XPA and xeroderma pigmentosum: TTD is a distinct condition from Xeroderma Pigmentosum (XP) caused by mutation in eight XP‐associated genes: XPA, XPD (ERCC2), XPB (ERCC3), XPF (ERCC4), XPG (ERCC5), XPC, XPE (DDB2), and a variant form related to the POLH gene.