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Sunlight may encourage skin cancer partly by impairing the immune system in genetically susceptible people, new studies suggest.

If so, a genetic test may one day be able to detect people at particular risk for skin cancer, researcher Dr. J. Wayne Streilein said.Researchers already have identified two possible susceptibility genes in mice and are looking for the human counterparts, he said.

Streilein described the mouse and human studies Monday at a science writers seminar sponsored by the American Cancer Society.

He said the work may hold implications for two kinds of skin cancer, basal cell and squamous cell, which appear in about 600,000 Americans a year. They are far less deadly than another form, melanoma. Strei-lein said researchers plan to see if the research applies to that disease as well.

The basic hypothesis of the research is that ultraviolet-B radiation, found in sunlight, impairs the immune system's operation in the upper layers of skin, and that this impairment prevents the immune system from recognizing and attacking cancerous skin cells.

Studies in mice show ultraviolet light can impair the immune system enough that their skin fails to react to an experimental chemical, he said. Studies also show that two genes govern whether a mouse is susceptible to the impairment, he said.

In 63 human volunteers who received a dose of ultraviolet-B light and then had a chemical irritant applied to their skin, 45 percent failed to mount the normal reaction, he said. So, like mice, people also differ in their susceptibility to this effect of ultraviolet-B light, he said.

The test with the chemical irritant is too cumbersome for widespread screening for susceptibility; such testing would be easier if based on the as-yet-unidentified human genes responsible, Streilein said.