DNA damage activates a cell-cycle checkpoint that prevents mitosis while DNA repair is under way(1). The protein Chk1 enforces this checkpoint by phosphorylating the mitotic inducer Cdc25 (refs 2-6), Phosphorylation of Cdc25 by Chk1 creates a binding site in Cdc25 for 14-3-3 proteins(5-8), but it is not known how 14-3-3 proteins regulate Cdc25. Rad24 is a 14-3-3 protein that is important in the DNA-damage checkpoint in fission yeast(9). Here we show that Rad24 controls the intracellular distribution of Cdc25. Elimination of Rad24 causes nuclear accumulation of Cdc25. Activation of the DNA-damage checkpoint causes the net nuclear export of Cdc25 by a process that requires Chk1, Rad24 and nuclear-export machinery. Mutation of a putative nuclear-export signal in Rad24 impairs the nuclear exclusion of Rad24, the damage-induced nuclear export of Cdc25 and the damage checkpoint. Thus, Rad24 appears to function as an attachable nuclear-export signal that enhances the nuclear export of Cdc25 in response to DNA damage.