frequency of cancerous changes in a cell. A defect in a DNA repair
gene called XP (Xeroderma pigmentosum) results in individuals who
are very sensitive to UV light and have a thousand-fold increase in the
incidence of all types of skin cancer. There are seven XP genes, whose
products remove DNA damage caused by UV light and other
carcinogens. Another example of a disease that is associated with loss
of DNA repair is Bloom syndrome, an inherited disorder that leads to
increased risk of cancer, lung disease, and diabetes. The mutated gene
in Bloom syndrome, BLM, is required for maintaining the stable
structure of chromosomes. Individuals with Bloom syndrome have a
high frequency of chromosome breaks and interchanges, which can
result in the activation of oncogenes.