Human cells from patients suffering with xeroderma pigmentosum (XP) characterized by extreme sensitivity to UV light and a high incidence of skin tumors fall into seven complementation groups, XPA to XPG, and are lacking a functional helicase, endonuclease, or lesion-recognizing protein involved in the initial steps during nucleotide excision repair (NER); a number of proteins involved in DNA repair are termed XPA to XPG depending on which one is defective in a particular complementation group of XP and include: (i) proteins involved in the recognition of (6-4) photoproducts (XPE) and of a broad range of lesions such as pyrimidine dimers (XPA); (ii) proteins that are DNA helicases and integral parts of the general transcription factor TFIIH functioning in both transcription and repair (XPB, XPD); (iii) endonucleases that perform the two incisions, the XPG incising six nucleotides (nt) to the 3' side from a photodimer and the ERCC1-XPF protein complex incising 22 nt to the 5' side of the lesion; and (iv) single-strand DNA-binding proteins (XPC). The ERCC6 helicase is largely responsible for coupling transcription to repair whereas XPC seems to be responsible for the repair of the inactive parts of the genome as well as for the repair of the nontranscribed strand in active genes. p53 recognizes insertion/deletion mismatches as well as free ends of DNA produced by ionizing radiation to arrest the cell cycle. Most of the human DNA repair proteins have their counterparts in both budding and fission yeasts and some of them also in E. coli evoking an evolutionary conservation of DNA repair pathways. Accumulation of mutations within repair genes in single cells followed by their escape from the immune surveillance and in clonal expansion may greatly contribute to the appearance and development of human cancers.