Replication fork collapse at single-strand DNA breaks threatens genome stability but how such forks are repaired and resolved has remained unclear. Here we replicate site-specific nicks with single or converging replication forks in Xenopuslaevis egg extracts. Collapse of a single fork generates a single-ended double-strand break (DSB) that undergoes homologous recombination to yield stable D-loops and end-to-end fusions, yet does not restart DNA synthesis. Single collapsed forks can also undergo extensive nucleolytic degradation, appearing to disassemble the sister fork through 'secondary collapse' events that resolve single-ended DSBs without engaging DSB repair. In contrast, semisynchronous convergent collapse generates a double-ended DSB that is primarily repaired through annealing-dependent DSB repair, completing DNA synthesis but generating precise deletions and templated insertions. These error-prone products are not detected following single-fork collapse. Our findings demonstrate that single and semisynchronous convergent collapsed forks elicit distinct repair outcomes.

Targeting replication-associated DNA repair mechanisms, including those regulated by PARP1/2 and PARG control of ADP-ribosylation is a powerful cancer therapeutic approach. However, the mechanisms by which PARG inhibition impacts DNA replication remain unclear. Here we combined isolation of proteins on nascent DNA (iPOND) with quantitative proteomics and functional assays to investigate replication fork dynamics upon acute PARG inhibition. We found that FET family proteins (FUS, EWS, and TAF15) are recruited to replication forks in a PAR-dependent manner, forming condensates that slow fork progression and promote fork reversal. FET proteins control fork dynamics in response to some, but not all, replication stresses. FUS inactivation leads to unrestrained fork progression via RECQ1 and PRIMPOL, increased single-stranded DNA gaps, genome instability, and synthetic lethality with BRCA1 deficiency. These findings reveal that FET protein condensates modulate replication stress responses, influencing genome stability and the cellular response to cancer therapeutics targeting PARylation pathways.

Accurate genome duplication requires tight-regulation of replication fork progression, and disruptions to this process are a major source of genomic instability, yet how fork dynamics are controlled during unperturbed S-phase remains unclear. We found replication forks elongate slowly in early S (ES) and faster in late S, independent of transcription or nucleotide availability. Elevated origin firing coupled with low TOP2A in ES generates torsional stress, causing replisome uncoupling, reduced fork speed, and basal ATR-CHK1 activation. Overexpression of TOP2A enhances fork speed and reduces replication stress in ES. Thus, TOP2A is a limiting replication factor during unperturbed ES, and basal ATR-CHK1 signaling is driven by transient replisome uncoupling. Also, TOP2A overexpression suppresses oncogene-driven replication stress. Given that TOP2A is frequently upregulated in cancers, it may function as a compensatory response to oncogene-induced replication stress. Together, these findings establish TOP2A as a central regulator of replication fork dynamics.

Abstract Flap endonuclease 1 (FEN1) is a structure-specific metallonuclease essential for Okazaki fragment maturation and DNA repair. We previously reported the discovery of BSM-1516, a potent and selective small-molecule FEN1 inhibitor that synergizes with PARP-targeted and other DNA damage response therapies and exhibits favorable in vivo pharmacokinetic properties. Pharmacologic inhibition of FEN1 increases its chromatin association, induces poly(ADP-Ribosyl)ation and ssDNA gaps, and is selectively cytotoxic to cells with homologous recombination deficiency. To characterize chromatin protein dynamics following FEN1 inhibition and identify potential pharmacodynamic (PD) biomarkers of target engagement, we employed isolation of Proteins On Nascent DNA (iPOND) coupled to mass spectrometry in proliferating cells treated with BSM-1516, alone or in combination with olaparib. FEN1 inhibition reproducibly enriched replication and DNA repair proteins, including FEN1, PARP1/2, LIG3, XRCC1, and CHD1L, reflecting PARP-dependent engagement of an alternative Okazaki fragment maturation pathway that was abrogated by co-treatment with olaparib. Orthogonal assays for chromatin-bound proteins confirmed selective enrichment of several iPOND-identified hits, establishing tractable PD biomarker candidates. Collectively, these findings delineate a proteomic signature of FEN1 inhibition at the replication forks and lay the groundwork for ongoing in vivo studies assessing these markers as indicators of target engagement in preclinical models. Citation Format: Jason Munguia, Sanjay Agarwalla, Dave Martin, Junhua Fan, Jack Schultz, Celeste Giansanti, David Cortez, David Puerta, Zachary Zimmerman, Konstantin Taganov. Proteomic profiling of FEN1 inhibition by BSM-1516 reveals chromatin-associated biomarkers for preclinical pharmacodynamic evaluation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 234.

Replication fork reversal helps maintain genomic stability during replication stress. F-box helicase 1 (FBH1) catalyzes fork reversal and is an SCF (SKP-CUL1-F-box) E3 ubiquitin ligase that limits RAD51 association with chromatin. Here, we show that preferential binding of SCFFBH1 to the lagging strand template at DNA fork structures stimulates helicase activity and is required for fork reversal. A cryo-EM structure of SCFFBH1 bound to DNA representing a stalled fork reveals an intimate interaction between FBH1 and the fork junction. Disruption of this interface severely curtails fork reversal in vitro and replication progression in cells, providing a model for how ssDNA translocation by FBH1 facilitates annealing of parental DNA by a fundamentally different mechanism than the fork remodelers SMARCAL, HLTF, and ZRANB3. The structure provides a model for SCFFBH1 disassembly of RAD51 filaments through translocation and ubiquitination, and implies that RAD51 is associated with the lagging strand at stalled forks. FBH1 is a DNA helicase and ubiquitin ligase that reverses stalled replication forks and limits RAD51 association with chromatin. Here, the authors describe the biochemical requirements for DNA unwinding and fork reversal activities and a cryo-EM structure of the SCFFBH1 complex bound to a DNA fork.

Replication fork collapse at single-strand DNA breaks (SSBs) poses a serious threat to genome stability. Using Xenopus egg extracts, we show that a replication fork encountering an SSB on either the leading- or lagging-strand template produces a single-ended double-strand break (seDSB). These broken ends are efficiently resolved through homologous recombination to yield D-loop intermediates and erroneous end-to-end fusions. Surprisingly, DNA synthesis downstream of an seDSB is highly inefficient. In contrast, when two forks converge at an SSB, they generate a double-ended DSB (deDSB) that efficiently completes DNA synthesis through double-strand break repair that is not dependent on homologous recombination. Leading, but not lagging, seDSBs can undergo extensive nucleolytic degradation that disassembles the divergent fork. These secondary collapse events efficiently resolve seDSBs but without completion of DNA synthesis. Moreover, PARP inhibition can enhance fork collapse at unmodified SSBs but not abasic site SSBs, contrary to expectations. Our findings distinguish end resolution from replication completion and demonstrate flexibility in how PARP inhibition affects fork collapse. ### Competing Interest Statement The authors have declared no competing interest.

Abasic sites are one of the most frequent forms of DNA damage that interfere with DNA replication. However, abasic sites exhibit complex effects because they can be processed into other types of DNA damage. Thus, it remains poorly understood how abasic sites affect replisome progression, which replication-coupled repair pathways they elicit, and whether this is affected by the template strand that is damaged. Using Xenopus egg extracts, we developed an approach to analyze replication of DNA containing a site-specific, stable abasic site on the leading or lagging strand template. We show that abasic sites robustly stall synthesis of nascent DNA strands but exert different effects when encountered on the leading or lagging strand template. At a leading strand AP site, replisomes stall ∼100 bp from the lesion until it is bypassed or a converging fork triggers termination. At a lagging strand abasic site, replisome progression is unaffected and lagging strands are reprimed downstream, generating a post-replicative gap, which is then bypassed. Despite different effects on replisome progression, both leading and lagging strand abasic sites rely on translesion DNA synthesis for bypass. Our results detail similarities and differences between how leading and lagging strand AP sites affect vertebrate DNA replication.

Abasic sites are frequent DNA lesions that interfere with replication and exert complex biological effects because they can be processed into other lesions. Thus, it remains poorly understood how abasic sites affect replisome progression, which repair pathways they elicit, and whether this depends on the template strand damaged. Using Xenopus egg extracts, we developed an approach to analyze replication of DNA containing a site-specific, stable abasic site on the leading or lagging strand template. We show that abasic sites robustly stall DNA synthesis but exert strand-specific effects. Leading strand abasic sites stall leading strands at the lesion, while lagging strands stall downstream at template-dependent positions. We conclude that replisomes uncouple at leading strand lesions, then stall due to additional template constraints. Synthesis restarts upon lesion bypass or when a converging fork triggers termination. In contrast, lagging strand abasic sites stall only lagging strands, indicating replisome progression was unaffected. Lagging strands reprime downstream, generating a post-replicative gap that is subsequently filled. Despite different effects on replisome progression, both leading and lagging strand abasic sites require translesion DNA synthesis for bypass. Our results reveal how strand-specific abasic sites differentially affect replication and demonstrate that uncoupled replisomes are susceptible to downstream template constraints.

Accurate sister chromatid segregation requires remodeling chromosome architecture, decatenation, and attachment to the mitotic spindle. Some of these events are initiated during S-phase, but they accelerate and conclude during mitosis. Here we describe SRBD1 as a histone and nucleic acid binding protein that prevents DNA damage in interphase cells, localizes to nascent DNA during replication and the chromosome scaffold in mitosis, and is required for chromosome segregation. SRBD1 inactivation causes micronuclei, chromatin bridges, and cell death. Inactivating SRBD1 immediately prior to mitotic entry causes anaphase failure, with a reduction in topoisomerase IIα localization to mitotic chromosomes and defects in properly condensing and decatenating chromosomes. In contrast, SRBD1 is not required to complete cell division after chromosomes are condensed. Strikingly, depleting condensin II reduces the severity of the anaphase defects in SRBD1-deficient cells by restoring topoisomerase IIα localization. Thus, SRBD1 is an essential genome maintenance protein required for mitotic chromosome organization and segregation.

Tumors with homologous recombination defects (HRD) due to mutations in BRCA1/2 genes or other genes associated with HR repair are typically sensitive to poly(ADP-ribose) Polymerase 1/2 inhibitors (PARPi), platinum-based drugs or other agents that target DNA repair pathways. Despite initial responsiveness to PARPi, many patients eventually experience disease progression. To that end, novel drug combination strategies involving PARPi plus other DNA replication and repair inhibitors have the potential to achieve more durable responses. Flap endonuclease 1 (FEN1) is a structure-specific metallonuclease that has been shown to be overexpressed in a variety of tumor types and has been reported to have many synthetic lethality partners, including PARP and BRCA2, making it an attractive target for the development of novel anticancer therapeutics. Utilizing an innovative library of metal-binding pharmacophores (MBPs) and a fragment-based drug discovery approach we identified a novel FEN1-selective chemical scaffold represented by BSM-1516 (IC50 of 7 nM and 460 nM in biochemical assays for FEN1 and EXO1, respectively; FEN1 cellular thermal shift target engagement assay EC50 of 24 nM). Examination of proteins on replication forks by iPOND-SILAC-MS in the presence of BSM-1516 revealed rapid enrichment of Okazaki fragment maturation (OFM) proteins FEN1/PCNA/LIG1, PARP1/2 enzymes, poly(ADP-ribose) binders (e.g. CHD1L) and alternative OFM pathway repair proteins XRCC1 and LIG3. Combination of BSM-1516 with inhibitors of PARP1/2 was strongly synergistic in vitro, enhanced their antiproliferative effect up to 100-fold (an effect not observed in normal fibroblasts) and led to robust activation of ssDNA break repair markers: phospho-Chk1(Ser345), phospho-RPA2(Ser33) and chromatin-bound RPA2. In vivo PK studies showed that BSM-1516 had oral bioavailability of 40% and T1/2 of 2.9 hours in mice. Safety of BSM-1516 was assessed in vitro in lineage-specific differentiation of human hematopoietic CD34+ progenitor cells and in vivo in mice at a daily dose of 120 mg/kg PO and 90 mg/kg IP for 7 days and revealed no signs of hematological toxicity. These collective data support further in vivo testing in PD and efficacy studies either as a single agent or in combination with PARPi. Jason Munguia, Sanjay Agarwalla, Dave Martin, Junhua Fan, Dave Lonergan, Celeste Giansanti, David Cortez, David Puerta, Zachary Zimmerman, Konstantin Taganov. Novel selective FEN1 nuclease inhibitor shows synergy with PARP-targeting drugs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5720.

G-quadruplexes (G4s) form throughout the genome and influence important cellular processes. Their deregulation can challenge DNA replication fork progression and threaten genome stability. Here, we demonstrate an unexpected role for the double-stranded DNA (dsDNA) translocase helicase-like transcription factor (HLTF) in responding to G4s. We show that HLTF, which is enriched at G4s in the human genome, can directly unfold G4s in vitro and uses this ATP-dependent translocase function to suppress G4 accumulation throughout the cell cycle. Additionally, MSH2 (a component of MutS heterodimers that bind G4s) and HLTF act synergistically to suppress G4 accumulation, restrict alternative lengthening of telomeres, and promote resistance to G4-stabilizing drugs. In a discrete but complementary role, HLTF restrains DNA synthesis when G4s are stabilized by suppressing primase-polymerase (PrimPol)-dependent repriming. Together, the distinct roles of HLTF in the G4 response prevent DNA damage and potentially mutagenic replication to safeguard genome stability.

DNA replication is remarkably accurate with estimates of only a handful of mutations per human genome per cell division cycle. Replication stress caused by DNA lesions, transcription-replication conflicts, and other obstacles to the replication machinery must be efficiently overcome in ways that minimize errors and maximize completion of DNA synthesis. Replication fork reversal is one mechanism that helps cells tolerate replication stress. This process involves reannealing of parental template DNA strands and generation of a nascent-nascent DNA duplex. While fork reversal may be beneficial by facilitating DNA repair or template switching, it must be confined to the appropriate contexts to preserve genome stability. Many enzymes have been implicated in this process including ATP-dependent DNA translocases like SMARCAL1, ZRANB3, HLTF, and the helicase FBH1. In addition, the RAD51 recombinase is required. Many additional factors and regulatory activities also act to ensure reversal is beneficial instead of yielding undesirable outcomes. Finally, reversed forks must also be stabilized and often need to be restarted to complete DNA synthesis. Disruption or deregulation of fork reversal causes a variety of human diseases. In this review we will describe the latest models for reversal and key mechanisms of regulation.

The authors have withdrawn this manuscript because they identified problems with how some figure panels were processed. Those experiments will be repeated before deposition of a new manuscript. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding authors.

Abstract Background Predicting clinical outcome in heart failure patients undergoing cardiac resynchronization therapy (CRT) remains challenging, necessitating improved risk stratification for CRT recipients. Previous studies have shown an association between large spatial peak and mean QRS-T angles and cardiovascular disease. Little is known, however, about their relation to clinical outcome following CRT. Purpose To investigate the association between spatial peak and mean QRS-T angles and long-term clinical outcome following CRT initiation. Methods All patients with native LBBB receiving CRT at a large-volume tertiary care center between 2015 and 2020 were retrospectively evaluated. Spatial peak and mean QRS-T angles were derived from digital pre- and post-CRT 12-lead ECGs that were processed using Glasgow algorithm and Kors’ regression transformation. The QRS-T angles and their change after CRT were analyzed in relation to the primary composite endpoint of heart failure hospitalisation or all-cause mortality using Cox regression analysis adjusted for clinical covariates (age, gender, CRT-P or CRT-D, secondary ICD indication, ischemic etiology, NYHA class, LVEF, diabetes, atrial fibrillation, baseline QRS duration, NT-proBNP, and eGFR). Results The study group comprised 250 patients (a median age [Q1–Q3] of 72.2 years [64.5–76.3]; 22% female; 58% New York Heart Association (NYHA) class III-IV; LVEF 27% [22–30]) who were followed over a median follow-up time of 4.5 years [3.2–5.8] after CRT implantation. The median post-CRT spatial peak and mean QRS-angles were 142° [116–160] and 152° [127–166], and the median angle was decreased by 16° [-0.6–41] and 14° [-0.4–38], respectively. Both a larger post-CRT mean QRS-T angle (HR 1.20, 95%CI 1.08-1.33, p=<.001) and peak QRS-T angle (HR 1.08, 95%CI 1.01-1.17, p=0.046) were associated with the primary endpoint. A larger reduction of mean QRS-T angle, but not peak QRS-T angle, was associated with a risk reduction (HR 0.87, 95%CI 0.79-0.95, p<0.003). In Kaplan-Meier analysis, patients with a post-CRT mean QRS-T angle above median had a higher risk of reaching the endpoint (log-rank p=0.002, Figure 1). Conclusion Our results show that a larger magnitude of the post-CRT spatial peak and mean QRS-T angles, and a larger reduction of the latter, are associated with long-term heart failure hospitalisation and death. These findings suggest that spatial QRS-T angle may have a potential role in refined risk stratification of CRT patients.Figure 1.K-M curve spatial mean QRS-T

8-oxoguanine (8-oxoG) is a common oxidative DNA lesion, which causes G>T substitutions that compose COSMIC single base substitution signature 18 (SBS18) in human cancers. Determinants of local and regional differences in 8-oxoG-induced mutability are currently unknown. To uncover factors influencing the topology of 8-oxoG-induced mutations, we assessed spontaneous and KBrO3-induced 8-oxoG mutagenesis in human cell lines. KBrO3 exposure produced a SBS18-like substitution spectrum and a distinct never-before reported INDEL signature that we also observed in human cancers. KBrO3-induced 8-oxoG lesions occurred with similar sequence preference as KBrO3-induced substitutions, indicating that the reactivity of specific reactive oxygen species (ROS) dictates the trinucleotide motif specificity for 8-oxoG-induced mutagenesis. While 8-oxoG lesions occurred relatively uniformly across chromatin states and nucleosomes, 8-oxoG-induced mutations occurred more frequently in more compact regions of the genome, within nucleosomal DNA, and at inward facing guanines within strongly positioned nucleosomes. Cryo-EM structures of OGG1 bound to nucleosomes indicate that these effects originate from OGG1’s ability to flip outward positioned 8-oxoG lesions into the catalytic pocket with only minor alterations to nucleosome structure, while inward facing lesions occluded by the histone octamer are unrecognized. Mutation spectra from cells with DNA repair deficiencies revealed a hierarchical DNA repair network limiting 8-oxoG mutagenesis in human cells, where OGG1– and MUTY-mediated BER is supplemented by replication-associated factors participating in tolerance of 8-oxoG or derived repair intermediates (i.e. Pol η and HMCES). Surprisingly, analysis of transcriptional asymmetry of KBrO3-induced mutations demonstrated transcription-coupled repair of 8-oxoG in Pol η-deficient cells. Thus, radical chemistry, chromatin structures, and DNA repair processes combine to dictate the oxidative mutational landscape in human genomes.### Competing Interest StatementThe authors have declared no competing interest.

Abstract RAD51 nucleoprotein filaments are central to maintaining genome stability, governing crucial processes like homology-directed double-strand break repair, replication fork reversal, and shielding replication forks from nucleases. The precise regulation of RAD51 filament formation and stability is critical for these functions, which suppress tumorigenesis and determine cellular responses to common cancer therapies. RADX is a pivotal regulator of RAD51 in the context of DNA replication, impacting replication fork reversal and fork stabilization. After identifying RADX as an RPA-related RAD51 regulator, we have worked to understand how it acts, thereby elucidating its role in genome stability and its influence on cancer cell responses to PARP inhibitors and chemotherapies. Genetically, RADX exhibits a dual role, capable of either inhibiting or promoting replication fork reversal based on the levels of replication stress. Biochemical studies show that RADX has inhibitory effects on RAD51 strand exchange and D-loop formation activities, achieved through direct binding to single-strand DNA and RAD51, along with the stimulation of RAD51 ATP hydrolysis. These activities collectively destabilize RAD51 nucleofilaments, opposing the stabilizing effects of BRCA2. Cells lacking RADX regulatory functions exhibit replication defects, DNA damage accumulation, reduced growth, and heightened sensitivity to DNA damage and replication stress. Structural analyses, including cryo-electron microscopy and mass photometry, revealed how RADX binds ssDNA and show it exists in multiple oligomeric states with a preference for trimers when bound to single-stranded DNA. Negative stain electron microscopy imaging supports a model wherein RADX functions by capping and restricting the growing ends of RAD51 filaments. In summary, our findings provide a comprehensive understanding of the regulatory mechanisms governing RAD51 nucleofilament dynamics by RADX, emphasizing its crucial role in coordinating replication fork stability and genome integrity. This knowledge not only contributes to the fundamental understanding of cellular processes but also offers insights into potential therapeutic interventions targeting RAD51-controlled pathways. Citation Format: Madison Adolph, Swati Balakrishnan, Walter Chazin, David Cortez. Mechanistic insights into how RADX regulates RAD51 nucleoprotein filaments to maintain genome stability and control replication stress responses [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr IA024.

Abstract8-oxoguanine (8-oxoG) is a common oxidative DNA lesion that causes G > T substitutions. Determinants of local and regional differences in 8-oxoG-induced mutability across genomes are currently unknown. Here, we show DNA oxidation induces G > T substitutions and insertion/deletion (INDEL) mutations in human cells and cancers. Potassium bromate (KBrO3)-induced 8-oxoGs occur with similar sequence preferences as their derived substitutions, indicating that the reactivity of specific oxidants dictates mutation sequence specificity. While 8-oxoG occurs uniformly across chromatin, 8-oxoG-induced mutations are elevated in compact genomic regions, within nucleosomes, and at inward facing guanines within strongly positioned nucleosomes. Cryo-electron microscopy structures of OGG1-nucleosome complexes indicate that these effects originate from OGG1’s ability to flip outward positioned 8-oxoG lesions into the catalytic pocket while inward facing lesions are occluded by the histone octamer. Mutation spectra from human cells with DNA repair deficiencies reveals contributions of a DNA repair network limiting 8-oxoG mutagenesis, where OGG1- and MUTYH-mediated base excision repair is supplemented by the replication-associated factors Pol η and HMCES. Transcriptional asymmetry of KBrO3-induced mutations in OGG1- and Pol η-deficient cells also demonstrates transcription-coupled repair can prevent 8-oxoG-induced mutation. Thus, oxidant chemistry, chromatin structures, and DNA repair processes combine to dictate the oxidative mutational landscape in human genomes.

Replication fork reversal is a fundamental process required for resolution of encounters with DNA damage. A key step in the stabilization and eventual resolution of reversed forks is formation of RAD51 nucleoprotein filaments on exposed single strand DNA (ssDNA). To avoid genome instability, RAD51 filaments are tightly controlled by a variety of positive and negative regulators. RADX (RPA-related RAD51-antagonist on the X chromosome) is a recently discovered negative regulator that binds tightly to ssDNA, directly interacts with RAD51, and regulates replication fork reversal and stabilization in a context-dependent manner. Here, we present a structure-based investigation of RADX’s mechanism of action. Mass photometry experiments showed that RADX forms multiple oligomeric states in a concentration-dependent manner, with a predominance of trimers in the presence of ssDNA. The structure of RADX, which has no structurally characterized orthologs, was determined ab initio by cryo-electron microscopy (cryo-EM) from maps in the 2 to 4 Å range. The structure reveals the molecular basis for RADX oligomerization and the coupled multi-valent binding of ssDNA binding. The interaction of RADX with RAD51 filaments was imaged by negative stain EM, which showed a RADX oligomer at the end of filaments. Based on these results, we propose a model in which RADX functions by capping and restricting the end of RAD51 filaments.

Abstract Background While the Medtronic Micra pacemaker provided a small device for leadless pacemaker implantation, the Aveir device allows chronic retrievability and mapping prior to fixation. We present the first Aveir leadless pacemaker implantations in a pediatric population. Purpose To describe the first retrievable leadless pacemaker implants in children. Methods Retrospective review of pediatric Aveir implants occurred between November 2022 and December 2022. All patients were implanted via internal jugular vein insertion utilizing the Aveir sheath and catheter deployment system with mid-atrial approach taken prior to insertion of the catheter across the tricuspid valve. All patients had initial mapping of capture threshold, impedance and R-wave noted prior to deployment. Indications for pacing included sinus pauses and intermittent atrioventricular block. Results Three patients underwent Aveir leadless pacemaker implantation with weights of 32.3kg, 44kg, and 51kg. Capture thresholds were all 0.75Volts@0.2 milliseconds with impedance range of 370-1110 ohms, and R-waves of 8-14 millivolts (mV). Follow-up at 3 months ranged from 0.5V@0.2ms to 1V@0.2ms with impedances ranging 400-700 ohms, and R-wave 7-14 mV. At VVI 50-55bpm, with pacing ranged from 5% to 14%, predicted longevities ranged from 20 to 24.5 years at 3 months follow-up. Conclusion(s) Retrievable leadless pacemaker implantation is feasible in children and may offer good longevities for intermittent pacing need.Aveir Leadless Pacemaker in RVAveir Leadless Pacemaker in RV