The invention provides compositions and methods used to inhibit USP1 deubiquitinase activity and to identify new inhibitors of USP1 deubiquitinase. The inhibitors can be used to treat or prevent cancer, bone marrow failure, and damage to cells or DNA resulting from genotoxic agents such as antineoplasic agents, including chemotherapeutic agents and radiation. The inhibitors include siRNA directed at inhibiting the expression of USP1 or UAF1, a protein which forms a heterodimeric complex with USP1. The inhibitors can be used to enhance cell survival if administered either before or after radiation exposure. Methods are also provided to enhance chemotherapy or radiotherapy of cancer and to enhance DNA repair. Transgenic knockout animals and knockdown cells are provided, whose USP1 expression is impaired.