Nitrogen starvation of Schizosaccharomyces pombe induces a differentiated state in which haploid cells mate and sporulate. esc1+, a newly isolated S.pombe cDNA that promotes this sexual differentiation, encodes a putative transcription factor with a helix-loop-helix (HLH) motif similar to those of the human MyoD and Myf-5 myogenic differentiation inducers. Disruption of esc1+ in wild-type cells leads to a decrease in the efficiency of sexual conjugation, an early step in sexual differentiation. The disruption was also able partially to substitute for cAMP, an inhibitor of differentiation, to suppress the lethal, constitutive differentiation induced by the pat1 mutation. Conversely, overexpression of this cDNA conferred partial resistance to cAMP-mediated inhibition of differentiation. Transcription from this novel gene was induced early in response to nitrogen starvation and is largely independent of the ste11+ gene product, which is required for the differentiation-specific expression of other genes. Thus, this MyoD/Myf-5-like protein appears to promote sexual differentiation by modulating responses to decreases in cAMP, a part of the nitrogen starvation signal that induces differentiation.