In Schizosaccharomyces pombe, an intrachromosomal crossover between the mating type (MT) expression locus and one of the silent donor cassettes is lethal due to the loss of the intervening L region. The region contains one essential gene, let1. This gene was cloned and sequenced. The deduced amino acid (aa) sequence of let1 shows extensive homologies with SUG1 from Saccharomyces cerevisiae. Significant homologies were also found with the human HIV transactivation modulators, MSS1 and TBP-1, as well as with subunit 4 of the mammalian 26 S protease. The data indicate that let1 is a member of a recently defined multigene family of ATPases.