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Xeroderma pigmentosum

Xeroderma pigmentosum is a genetic disorder of DNA repair in which the body's normal ability to fix mutations caused by UV light is disabled. This leads to multiple basaliomas and other skin malignancies at a young age. Therefore in very severe cases it is necessary to avoid all sunlight. more...

X-linked adrenal...
X-linked ichthyosis
X-linked severe combined...
Xeroderma pigmentosum
XX male syndrome
XY Female


There are eight types.

  • Dominant Type (OMIM 194400)
  • Type A, I, XPA, Classical Form (OMIM 278700)
  • Type B, II, XPB (OMIM 133510)
  • Type C, III, XPC (OMIM 278720)
  • Type D, IV, XPD (OMIM 278730)
  • Type E, V, XPE (OMIM 278740)
  • Type F, VI, XPF (OMIM 278760)
  • Type G, VII, XPG (OMIM 278780)


Damage to DNA is a frequent occurrence, and the body has mechanisms to fix it by cutting out the damaged segment and replacing the resulting gap in the DNA with the correct base pairs from the opposing strand ("nucleotide excision repair"). The enzymes DNA polymerase and ligase fulfil this role. The most common defect in xeroderma pigmentosum is an inherited defect that alters the nucleotide excision repair enzymes, and hinders their functionality. Therefore, DNA damage is not readily fixed.


The most important part of managing the condition is reducing exposure to the sun.

The number of keratoses can be reduced with Isotretinoin () (though there are significant side-effects.) Existing keratoses can be treated using cryotherapy or fluorouracil. ().

In popular culture

A notable fictional character who sufferers XP is Christopher Snow featured in the novels Fear Nothing and Seize the Night by Dean R. Koontz.

The film The Others features characters who suffer from XP.

Late in 2005, ABC's Extreme Makeover: Home Edition gave kids with XP a once in a lifetime opportunity, a night at Disney World. Disney World opened for a night just for kids with XP. Footage from this event aired as part of an Extreme Makeover holiday special on December 11, 2005.


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From Gale Encyclopedia of Medicine, 4/6/01 by J. Ricker Polsdorfer


Photosensitivity is any increase in the reactivity of the skin to sunlight.


The skin is a carefully designed interface between our bodies and the outside world. It is infection-proof when intact, nearly waterproof, and filled with protective mechanisms. Sunlight threatens the health of the skin. Normal skin is highly variable in its ability to resist sun damage. Natural skin pigmentation is its main protection. The term photosensitivity refers to any increase beyond what is considered normal variation.

Causes & symptoms

There are over three dozen diseases, two dozen drugs, and several perfume and cosmetic components that can cause photosensitivity. There are also several different types of reaction to sunlight--phototoxicity, photoallergy, and polymorphous light eruption. In addition, prolonged exposure to sunlight, even in normal skin, leads to skin aging and cancer. These effects are accelerated in patients who have photosensitivity.

Diseases of several kinds increase skin sensitivity.

  • Phototoxicity is a severely exaggerated reaction to sunlight caused by a new chemical in the skin. The primary symptom is sunburn, which is rapid and can be severe enough to blister (a second degree burn). The chemicals associated with phototoxicity are usually drugs. The list includes several common antibiotics--quinolones, sulfonamides, and tetracyclines; diuretics (water pills); major tranquilizers; oral diabetes medication; and cancer medicines. There are also some dermatologic drugs, both topical and oral, that can sensitize skin.
  • Photoallergy produces an intense itching rash on exposure to sunlight. Patients develop chronic skin changes (lichen simplex) as a result of scratching. Some of the agents that cause phototoxicity can also cause photoallergy. Some cosmetic and perfume ingredients, including one of the most common sunscreens--para-amino benzoic acid (PABA)--can do this.
  • Polymorphous light eruption resembles photoallergy in its production of intensely itching rashes in sunlight. However, this condition lessens with continued light exposure, and so is seen mostly in the spring. Also, there does not seem to be an identifiable chemical involved.
  • A hereditary disease called xeroderma pigmentosum includes a defect in repair mechanisms that greatly accelerates skin damage from sunlight.
  • A family of metabolic diseases called porphyrias produce chemicals (porphyrins) that absorb sunlight in the skin and thereby cause damage.
  • Albinos lack skin pigment through a genetic defect and are thus very sensitive to light.
  • Malnutrition, specifically a deficiency of niacin known as pellagra, sensitizes the skin.
  • Several diseases like acne, systemic lupus erythematosus, and herpes simples (fever blisters) decrease the resistance of the skin to sun damage.


The pattern of appearance on the skin, a history of drug or chemical exposure, and the timing of the symptoms often suggests a diagnosis. A skin biopsy may be needed for further clarification.


Removal of the offending drug or chemical is primary. Direct sunlight exposure should be limited. Some people must avoid sunlight altogether, while others can tolerate some direct sunlight with the aid of sunscreens.


A sunscreen with an SPF of 15 or greater protects most skin from damage. Protective clothing such as hats are highly recommended in addition.

Key Terms

Surgical removal of tissue for examination.

Further Reading

For Your Information


  • Bickers, David R. "Photosensitivity and other reactions to light." In Harrison's Principles of Internal Medicine. edited by Anthony S. Fauci, et al. New York: McGraw-Hill, 1998, pp. 329-333.
  • Harber, Leonard C. "Abnormal responses to ultraviolet radiation: drug induced photosensitivity." In Dermatology in General Medicine, edited by Thomas B. Fitzpatrick, et al. New York: McGraw-Hill, 1993 pp. 1677-1689.

Gale Encyclopedia of Medicine. Gale Research, 1999.

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