API5 (apoptosis inhibitor 5) is an antiapoptotic factor that may have a role in protein assembly. API5 negatively regulates ACIN1. By binding to ACIN1, API5 suppresses ACIN1 cleavage from CASP3 and ACIN1-mediated DNA fragmentation. API5 is also known to efficiently suppress E2F1-induced apoptosis. Its depletion enhances the cytotoxic action of the chemotherapeutic drugs.