The XPA gene encodes a 31 kDa zinc metalloprotein that preferentially binds to DNA damaged by UV radiation and chemical carcinogens and is required for the incision step during nucleotide excision repair. The XPB and XPD genes encode DNA helicases involved in several DNA metabolic pathways, including DNA repair and transcription, and the XPG gene product is an endonuclease that cuts on the 3' side of a DNA lesion during nucleotide excision repair. Molecular defects in the XP variant (XPV) group maintain normal excision repair, yet they result in a substantial reduction in the ability to synthesize intact daughter DNA strands during DNA replication following DNA damage.