Xeroderma pigmentosum is a genetic disease characterized by such extraordinary sensitivity to sunlight that it results in the development of skin cancer at a very early age. It is abbreviated as XP. Children with XP disease can only play outdoors safely after nightfall. XP is due to defective repair of damage done to DNA by ultraviolet (UV) light. Whereas normal people can repair UV-induced damage by inserting new bases into the DNA, people with XP cannot. A person with XP develops severe sunburn and eye irritation within minutes of exposure to sunlight. Other features of XP include very dry skin (xeroderma), blisters on the skin, heavy freckling, and dark spots on the skin. XP is inherited in an autosomal recessive manner. Defects in multiple genes have been identified that lead to XP. Avoiding UV light and using the highest level of sunscreen possible when exposure cannot be avoided helps prevent complications.
Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.