We have isolated a conditional lethal mutant mts3 in the fission yeast Schizosaccharomyces pombe which at the permissive temperature is resistant to the mitotic poison MBC and at the restrictive temperature is defective in metaphase to anaphase transition. The predicted amino acid sequence of mts3+ is 36% identical with the budding yeast gene NIN1. NIN1 cloned into a fission yeast expression vector can rescue both mts3 temperature-sensitive and null alleles demonstrating that NIN1 is the budding yeast homologue of the fission yeast mts3+ gene. The phenotype of the mts3 null is identical with the mts3 ts mutant demonstrating that the phenotype of the mts3 ts mutant is due to loss of mts3+ function. The deduced amino acid sequences of both mts3+ and NIN1 show homology to peptide sequences obtained from subunit 14 of the 26 S protease purified from bovine or human cells.