Gadd45a, gadd45b and gadd45g (Gadd45/MyD118/CR6) are genes that are rapidly induced by genotoxic stress. However, the exact function of Gadd45 proteins in the response of mammalian cells to genotoxic stress is unclear. Here, advantage was taken of gadd45a-and gadd45b-deficient mice to determine the role gadd45a and gadd45b play in the response of bone marrow (BM) cells to genotoxic stress. BM cells from gadd45a-and gadd45b-deficient mice were observed to be more sensitive to ultraviolet radiation chemotherapy (UVC), VP-16 and daunorubicin (DNR)-induced apoptosis compared to wild-type (wt) cells. The increased apoptosis in gadd45a-and gadd45b-deficient cells was evident also by enhanced activation of caspase-3 and poly-ADP-ribose polymerase cleavage and decreased expression of c-inhibitor of apoptotic protein-1, Bcl-2, Bcl-xL compared to wt cells. Reintroduction of gadd45 into gadd45-deficient BM cells restored the wt apoptotic phenotype. Both gadd45a-and gadd45b-deficient BM cells also displayed defective G2/M arrest following exposure to UVC and VP-16, but not to DNR, indicating the existence of different G2/M checkpoints that are either dependent or independent of gadd45. Taken together, these findings identify gadd45a and gadd45b as antiapoptotic genes that increase the survival of hematopoietic cells following exposure to UV radiation and certain anticancer drugs.