Loss of the p12 subunit of DNA polymerase delta leads to a defect in HR and sensitization to PARP inhibitors.

•p12-knockout generates a HR deficiency phenotype.•Pol δ4 is required for full expression of cellular HR activity.•p12 knockout cells are sensitized to PARP inhibitors.•p12 expression is a potential biomarker for the use of PARP inhibitors.•p12 deficiency may have a role in SCLC tumorigenesis.