The ran1+ gene of the fission yeast Schizosaccharomyces pombe is a negative regulator of both sexual conjugation and meiosis. The nucleotide sequence of the gene has been determined and contains a region of open reading frame (ORF) capable of encoding a protein of 52,000 daltons. S1 nuclease analysis of ran1+-encoded RNA showed that the ORF was spanned by an uninterrupted transcript. A fragment of DNA containing the entire ran1+ gene was expressed in a bacterial expression vector and found to encode the expected product of 52,000 daltons. The putative ran1+ gene product shares significant sequence homology with known protein kinases. The level of the ran1+ transcript was similar in vegetative and meiotic cells suggesting that the ran1+ protein product rather than its transcript is regulated during sexual differentiation.