Xeroderma Pigmentosum (or XP) is a rare inherited disease affecting both males and females.  It causes a person to be extremely sensitive to the damaging effects of ultraviolet radiation.  Undiagnosed and untreated, XP can lead to the early onset of skin cancer and blindness.  In addition, approximately 20% of the people with XP develop progressive neurological disease.XP is an autosomal recessive disease caused by mutations in genes that are critical for DNA repair. The body must repair DNA when it is damaged by harmful external agents such as ultraviolet radiation, X-rays and chemicals. There are at least eight genes that are critical for this DNA repair process. If a person is born with any of the eight genes not functioning correctly, he or she will develop the symptoms of XP. The genes for XP are: XP-A, XP-B, XP-C, XP-D, XP-E, XP-F, XP-G, and variant XP. Prior to the discovery of the XP genes themselves, the term "complementation groups" was used to describe the laboratory variations seen in the cells of XP patients. The cells were categorized according to the capacity of the cells to repair DNA after they had been damaged by ultraviolet radiation in the laboratory. Mutations in the A, C, D, and Variant genes make up over 90% of XP cases