XPA and xeroderma pigmentosum: It is well known that there are five main DNA repair pathways with more than 130 genes, of which the nucleotide excision repair (NER) pathway plays a crucial DNA repair mechanism by removing various DNA lesions caused by UV radiation and some chemical agents, such as bulky adducts, cross links, oxidative DNA and alkylating damage and thymidine dimers.6-8 It is reported that xeroderma pigmentosum (XP) complementation groups in human, XPA to XPG, represent the rate-limiting proteins in the NER pathway.9