Human damaged DNA-binding protein (DDB) is a heterodimer of p48/DDB2 and p127/DDB1 subunits. Mutations in DDB2 are responsible for Xeroderma Pigmentosum group E, but no mutants of mammalian DDB1 have been described. To study DDB1, the Schizosaccharomyces pombe DDB1 sequence homologue (ddb1(+)) was cloned, and a ddb1 deletion strain was constructed. The gene is not essential; however, mutant cells showed a 37% impairment in colony-forming ability, an elongated phenotype, and abnormal nuclei. The ddb1Delta strain was sensitive to UV irradiation, X-rays, methylmethane sulfonate, and thiabendazole, and these sensitivities were compared with those of the well characterized rad13Delta, rhp51Delta, and cds1Delta mutant strains. Ddb1p showed nuclear and nucleolar localization, and the aberrant nuclear structures observed in the ddb1Delta strain suggest a role for Ddb1p in chromosome segregation.