Mesothelin is a tumor-differentiation antigen discovered more than 30 years ago, although the development of novel and effective therapies targeting mesothelin still draws considerable interest in the oncology drug discovery and development field.1,2,3 In the December 19th issue of Molecular Therapy Oncolytics, a new study by Sun and colleagues reports the isolation and characterization of a novel human antibody heavy-chain variable (VH) domain named 3C9, targeting mesothelin as an antibody-drug conjugate (ADC).