Cell cycle checkpoints are essential for the maintenance of genomic stability in response to DNA damage. We demonstrated recently that GADD45, a DNA damage-inducible protein, activates a G2/M checkpoint induced by either UV radiation or alkylating agents. GADD45 can interact in vivo with the G2 cell cycle-specific kinase, Cdc2, proliferating cell nuclear antigen (PCNA), and the cell cycle kinase inhibitor p21(waf1). The ability of GADD45 to induce a G2/M arrest may be caused in part by the inhibition of Cdc2 kinase activity. Here, we report the identification of a region of GADD45 that is involved in this G2/M checkpoint. Mutants of GADD45 that lacked either the first 35 or the last 80 residues still retained an ability to induce G2/M arrest. A mutant with a deletion of the central region (residues 50-76), which is conserved in the family members GADD45β and GADD45γ, lacked such activity. This mutant also lacked an ability to bind to Cdc2, PCNA, and p21(waf1) in vivo. Consistently, either GADD45β or GADD45γ bind to Cdc2 in vivo. However, unlike GADD45, neither GADD45β nor GADD45γ inhibited the Cdc2 kinase or induced G2/M arrest. The unique effect of GADD45 may be caused by the presence of a region containing DEDDDR residues. Alanine substitutions in the region abolished GADD45 induction of a G2/M arrest and its inactivation of the Cdc2 kinase but not its binding to Cdc2, PCNA, or p21(waf1). Therefore, the binding of GADD45 to Cdc2 was insufficient to induce a G2/M arrest, and additional activity contributed by the DEDDDR residues may be necessary to regulate the G2/M checkpoint.