Xeroderma pigmentosum (XP) is a sun-sensitive, cancer-prone genetic disorder characterized by a defect in nucleotide excision repair. The human nucleotide excision repair and transcription gene ERCC2 is able to restore survival to normal levels after exposure to UV light in XP complementation group D cells. No enhancement of UV survival is seen in groups C, E, F, or G. XP-CS-2 cells are complemented by ERCC2, confirming the reassignment to group D of this combined XP/Cockayne's syndrome patient. Nucleotide sequence analysis of the ERCC2 cDNA from five XP group D cell strains [XP6BE(SV40), XP17PV, XP102LO, A31-27 (a HeLa/XP102LO hybrid), and XP-CS-2] revealed mutations predominantly affecting previously identified functional domains. The mutations include base substitutions resulting in amino acid substitutions, deletions due to splicing alterations, and defects in expression. XP6BE(SV40), XP17PV, XP102LO, and A31-27 all have one allele with an Arg683 to Trp substitution within the putative nuclear location signal. The genetic disorder trichothiodystrophy (which is not cancer-prone) can also result from mutations in the ERCC2 gene, some of which are the same as those found in XP-D. The various clinical presentations can be correlated with the particular mutations found in the ERCC2 locus.