Fission yeast requires nutritional starvation to switch the mitotic cell cycle to sexual differentiation, but sam mutants, of which we had isolated nine alleles, mate without the starvation condition. These mutants are useful for understanding the mechanism underlying the way cells sense nutritional starvation and change the cell cycle. To identify the sam allele, we first sought phenotypes other than the original sam phenotype. We found that all nine sam mutants were sensitive to 1 M KCl, that sam2, sam3, sam4 and sam9 were sensitive to 0.1 M CaCl(2), and that only the sam4 mutant was sensitive to 150 J/m(2) UV. This peculiar phenotype of sam4 suggested to us that sam4 might be an allele of rad24, which encodes a 14-3-3 protein. In fact, the Rad24 protein disappeared in sam4 and the rad24 mRNA was not transcribed in sam4. In addition, the mutation that changed Gln to a stop codon was found in the rad24 locus of sam4. Hence we concluded that sam4 is an allele of rad24. We also found that over-expression of rad24 or rad25 (a paralog of rad24) has a suppressive effect on sam1, and that sam1 was not an allele of rad24 nor rad25. Thus 14-3-3 proteins are deeply involved in the switching of the mitotic cell cycle to the sexual differentiation of fission yeast.