Abstract: Nucleotide excision repair is a major pathway for repairing UV light-induced DNA damage in most organisms. Using insertional mutagenesis, we isolated a UV-sensitive mutant of Chlamydomonas reinhardtii that is blocked in the excision of cyclobutane pyrimidine dimers. The mutant is also sensitive to the alkylating agent, methyl methanesulphonate. We have cloned REX1, a novel gene that rescues the mutant. The gene is unusual in a eukaryotic organism in that it is predicted to encode two different proteins, a small protein (8.9 kDa) and a larger protein (31.8 kDa). Neither protein is homologous to known DNA repair proteins. Partial complementation is achieved with subclones of the gene encoding only the 8.9-kDa protein. The 8.9-kDa protein has homologues in many organisms including Saccharomyces cerevisiae, Arabidopsis, and humans. The 31.8-kDa protein appears to be less conserved. These findings may be of general importance for DNA repair in other organisms.