The human DNA excision repair protein ERCC-1 exhibits homology to the yeast RAD10 repair protein and its longer c-terminus displays similarity to parts of the E.coli repair proteins uvrA and uvrC. To study the evolution of this 'mosaic' ERCC-1 gene we have isolated the mouse homologue. Mouse ERCC-1 harbors the same pattern of homology with RAD10 and has a comparable C-terminal extension as its human equivalent. Mutation studies show that the strongly conserved C-terminus is essential in contrast to the less conserved N-terminus which is even dispensible. The mouse ERCC-1 amino acid sequence is compatible with a previously postulated nuclear location signal and DNA-binding domain. The ERCC-1 promoter harbors a region which is highly conserved in mouse and man. Since the ERCC-1 promoter is devoid of all classical promoter elements this region may be responsible for the low constitutive level of expression in all mouse tissues and stages of embryogenesis examined.