Xeroderma pigmentosum is produced as a result of a defect in

It is derived from the Greek terms xeres = dry and derma = skin. It is an autosomal recessive condition. Defect lies in the NER (nucleotide excision repair) mechanism. There are seven XP genes (A to G) necessary for NER mechanism in humans. Mutation in any one of them may lead to the condition XP. UV light (sun light) causes formation of thymine dimers, where covalent bonds are formed between adjacent thymine residues. Since repair mechanism is not operating, these mutations are accumulated, leading to cancer. There is sensitivity to ultraviolet rays; sunlight causes blisters on the skin. Avoiding sunlight and using sunscreen ointment will be beneficial. Patients with XP have a 1000-fold greater chance of developing skin cancer than do normal persons. Death usually occurs in the second decade of life due to squamous cell carcinoma of skin. Prenatal diagnosis of XP is possible.Ref: DM Vasudevan, page no: 479