We examined several yeast strains with different mutations in the essential SSL2 (Suppressor of Stem Loop, also called RAD25) gene for their ability to remove cyclobutane pyrimidine dimers from expressed genes, and from the genome overall. The SSL2 protein has a high degree of amino acid sequence identity to the protein encoded by the human ERCC3 gene (Gulyas, K. D., and Donahue, T. F. (1992) Cell 69, 1031-0142). The mutant allele SSL2-XP encodes a protein resembling the mutated ERCC3 protein from UV-sensitive human cells belonging to xeroderma pigmentosum complementation group B and Cockayne's syndrome (CS) complementation group C (Weeda, G., van Ham, R. C. A., Vermeulen, W., Bootsma, D., van der Eb, A. J., and Hoeijmakers, J. H. J. (1990) Cell 62, 777-791; Gulyas and Donahue, 1992). The SSL2-XP allele confers UV sensitivity on yeast strain KG119. We found that the biochemical basis for the UV sensitivity of KG119 is a complete deficiency in the removal of cyclobutane pyrimidine dimers from the overall genome as well as a deficiency in transcription-coupled repair. This is the first analysis of the DNA repair defect responsible for the UV sensitivity of cells carrying the SSL2-XP allele, and it documents the similarity of the defect to that associated with XP-B/CS-C, and the difference between this defect and that in cells belonging to CS complementation groups A and B.