Differences in patterns of complementation of the more common groups of xeroderma pigmentosum: possible implications.

Xeroderma pigmentosum of groups A, C and D shows complementation differing in kinetics, dependence on the dose of wild-type alleles and dependence on protein synthesis. Such differences suggest that XP-A, -C and -D carry mutations at different loci. The product of the first of these loci (factor A) is present in significant excess in normal fibroblasts, seems to turn over rapidly and may be a dimer or higher polymer. The products of the other two loci (factors C and D) do not seem to be present in significant excess in the cytoplasm of normal fibroblasts, but factor C may accumulate abnormally in XP-D. Factors C and D turn over slowly (D more than C) and they do not move freely from the cell nucleus. Factors A and C, at least, seem to act directly and not via gene regulation.