Defects in human DNA repair proteins can give rise to the autosomal recessive disorders xeroderma pigmentosum (XP) and Cockayne's syndrome (CS), sometimes even together. Seven XP and three CS complementation groups have been identified that are thought to be due to mutations in genes from the nucleotide excision repair pathway. Here we isolate frog and human complementary DNAs that encode proteins resembling RAD2, a protein involved in this pathway in yeast. Alignment of these three polypeptides, together with two other RAD2 related proteins, reveals that their conserved sequences are largely confined to two regions. Expression of the human cDNA in vivo restores to normal the sensitivity to ultraviolet light and unscheduled DNA synthesis of lymphoblastoid cells from XP group G, but not CS group A. The XP-G correcting protein XPGC is generated from a messenger RNA of approximately 4 kilobases that is present in normal amounts in the XP-G cell line.