To date, nine genes have been associated with XP: XPA, ERCC3 (XPB), XPC, ERCC2 (XPD), DDB2 (XPE), ERCC4 (XPF), ERCC5 (XPG), ERCC1, and POLH (XP-V). Here, ERCC1 is linked to xeroderma pigmentosum.