Cells derived from patients suffering from xeroderma pigmentosum either lacks or have reduced DNA repair capacity due to genetic mutations in several components of the NER. The XPA complementation type represents the most severe phenotype, because the XPA gene is the most crucial component in the DNA repair process and, thus, cells lacking the XPA gene are completely deficient in NER [16]. Here, XPA is linked to xeroderma pigmentosum.