Summary: Four rare genetic diseases, xeroderma pigmentosum (XP), Cockayne syndrome (CS), the XP/CS complex and trichothiodystrophy (TTD) have defective DNA excision repair although only XP has increased cancer susceptibility. We plan to perform careful clinical examination of selected patients with XP, XP/CS, CS, or TTD and follow their clinical course. We will obtain tissue (skin, blood, hair, buccal swab...