GADD45 (Growth arrest and DNA damge) regulates cell growth following exposure to diverse stimuli. It has been shown that, mice lacking the gadd45a gene exhibit genomic instability and increased carcinogenesis, but the exact role of the gadd45 family genes still remains unclear. In this study we have aimed at determining the effect of gadd45a or gadd45b deficiency on the response of bone marrow derived myeloid cells to genotoxic stress agents by using gadd45a or gadd45b null mice. We have found that myeloid progenitor cells from gadd45a or gadd45b-null mice are more sensitive to ultraviolet-radiation (UV), VP-16 or daunorubicin induced apoptosis. Introduction of wild-type gadd45 into gadd45-deficient bone marrow cells restored the wild-type apoptotic phenotype. In-vitro colony formation following stress responses has shown that bone marrow cells from gadd45a or gadd45b-deficient mice have a decreased ability to form haematopoetic colonies. Gadd45a or gadd45b-deficient bone marrow cells also displayed defective G2/M cell cycle checkpoint following exposure to either UV and V-16 but were still able to undergo G2/M arrest following exposure to daunorubicin, indicating the existence of different G2/M checkpoints in response to these anticancer agents. Taken together these findings identify gadd45a or gadd45b as anti-apoptotic gene(s), and suggests that the absence of gadd45a or gadd45b results in higher susceptibility of haematopoetic cells to UV radiation and certain anticancer drugs.