Xeroderma Pigmentosum (abbreviated as XP) is an autosomal recessive genetic disorder. XP is characterized by abnormal pigmentation on the Sun exposed areas of the skin ( thus the name Pigmentosum), extreme sensitivity of the skin to Sunlight and marked predisposition to skin cancer. In some cases growth and mental retardation are also associated with this syndrome.

Cockayne Syndrome abbreviated as (CS) is also a human genetic disorder and is closely related to XP. CS is associated with photosensitivity but not necessarily skin cancer. Symptoms of CS include profound growth retardation, retinopathy, cataracts, large sunken eyes and thin prominent nose. These patients have prematurely aged appearance.

In comparison to cells from normal individuals, cells derived from XP and CS individuals are highly sensitive to Ultraviolet (UV) radiations. The biochemical defect in XP and CS cells has been found to be in the repair of the UV-induced DNA damage.