Unscheduled DNA synthesis, photoreplication repair capacity, and photoreactivating enzyme levels were examined in cells of individuals of a family with one case of XP and otherwise clinically normal parents. The patient's parents were first cousins. The activity of three paths of DNA repair was depressed in the XP cells. The clinically normal parents showed normal levels of unscheduled DNA synthesis as well as postreplication repair, however their photoreactivating enzyme level was as low as 30% of normal levels.

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