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 swabs) for laboratory examination of DNA repair and for genetic analysis. We hope to be able to correlate these laboratory abnormalities with the clinical features to better understand the mechanism of cancer prevention by DNA repair. Patients will be offered counseling and education for cancer control.