Schizosaccharomyces pombe (Sp) rad23-1 mutant cells are extremely sensitive to UV light and ionizing radiation. A genomic DNA fragment that contains wild-type (wt) rad23 has been cloned. The DNA sequence of this cloned gene has been determined and was found to be identical to the previously characterized mating-type switching/radioresistance gene, swi10. Complementation tests between rad23-1 and swi10-154 mutant cells exclusively produce UV-sensitive progeny and confirm that these two genes are allelic. The DNA sequences of rad23-1 and swi10-154 reveal that each contains a single, unique point mutation. In rad23-1, Glu231 changes to a stop codon, resulting in the production of a truncated protein. In swi10-154, a G to A transition mutation is within a splice consensus sequence for intron 1. Therefore, the corresponding mRNA is incapable of being processed appropriately.