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These days, when someone says "XP", (granted, in typical every day conversation, it doesn't come up that often), I immediately think Experience Points, which I associate with E2 which invariably causes me to wonder why I'm wasting my time doing something other than noding.

But there was a time, not so long ago, when someone said "XP", I would associate it with The Chris Snow Trilogy by Dean Koontz. Christopher Snow, the primary character in the series suffers from a genetic condition known as Xeroderma Pigmentosum, or XP for short.

By necessity, the books are set almost exclusively during the night in the town of Moonlight Bay, California, because Snow's condition renders him extremely vulnerable to the sun's ultraviolet rays. On those rare occasions when Snow must venture out of doors during the day, he wears protective clothing that covers every speck of skin, and wears dark, almost opaque, sunglasses.

When I began reading the trilogy, I had never heard of XP. It is a very rare human autosomal recessive disease - there are less than one thousand cases worldwide - and due to it's non-sensational nature, the media has no interest in it. I don't run in circles where conditions and anomalies are discussed, so I was completely ignorant of the disease. As I read I assumed the disease was a plot device in order to give the dark feeling of the book a basis. As it turned out Chris's Xeroderma Pigmentosum had little to do with the plot on any meaningful level. When I got to the end of the book, there was an argument by Koontz where he explained that he didn't make up XP, and, indeed Chris Snow is based on a real person.

XP doesn't effect skin color, and sufferers are not albinos. On a practical level, XP forces bearers of the defect to avoid bright light if possible, and avoid sunlight at all costs. The defect retards - or more accurately prevents - the body's ability to repair itself from damage caused by ultraviolet radiation. Normal skin repairs damage by sunlight to DNA by inserting new bases into the DNA, however, sufferers of XP lack this capacity, or have a reduced capacity, and the damage suffered is cumulative. The first symptoms of XP usually begin to show up by age one or two.

Symptoms can include diffuse erythema, severe sunburn caused by small amounts of sunlight, even with sunblock, and freckle-like spots to sun exposed skin. There are several indicators for the disease including blindness, deafness, developmental disabilities, dwarfism and mental retardation.

Later, as the disease progresses, skin atrophy, telangiectasias, and mottled skin color appears. Finally the skin gains tumors, squamous cell carcinoma, melanoma, basal cell carcinoma and fibrosarcoma (all cancers). In severe cases, these symptoms may appear by age five.

There are several severity levels of XP, grouped by letters A through G. Group A has the lowest tolerance and the least DNA repair capability. Therefore they suffer the worst symptoms the quickest.

XP was named in 1882 by Dr. Kaposi who named it for the dry pigmented skin of sufferers.

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