Figure 1: Three disorders for nine genes. Mutations in ten genes are responsible for the xeroderma pigmentosum (XP), the trichothyodystrophy (TTD) or the Cockayne syndrome (CS). XPA, XPC, XPE and XPF are only involved in XP; CSA and CSB are only involved in CS; XPG is involved in pure XP or in an combined XP/CS syndrome; XPB and XPD are involved in TTD, XP, or in a combined XP/CS syndrome. TTDA is only involved in TTD.