Xeroderma pigmentosum (XP) is a human autosomal recessive disease and is characterized by seven complementation groups from A through G. XPD/ERCC2 (Rad3 in Saccharomycescerevisiae), an ATP-dependent helicase, belongs to RAD3/XPD helicase subfamily (1). The gene discussed is WRN; the disease is xeroderma pigmentosum.