Xeroderma pigmentosum (XP) is a rare, inherited disorder of defective DNA repair, which can offer insights into the mechanism of cancer development and neurologic degeneration.

This long-term study of 106 XP patients at the National Institutes of Health found a 10,000-fold increase in non-melanoma skin cancer (NMSC) and a 2,000-fold increase in melanoma following sun exposure. The NMSC occurred at a younger age than melanomas in XP patients. This age of onset is the reverse of what occurs in the general population, suggesting differences in carcinogenic mechanisms between melanoma and NMSC.

Many XP patients have progressive neurologic degeneration, and a surprisingly high mortality, possibly due to unrepaired DNA damage from other agents. (By Dr. Kenneth H. Kraemer, http://jmg.bmj.com/content/early/2010/11/19/jmg.2010.083022.abstract?papetoc )