We have cloned human xeroderma pigmentosum group A complementing (XPAC) cDNA that encodes a "zinc finger" protein with a predicted size of 31 kDa. To detect the xpac protein in cells, we raised antibody against a recombinant human xpac protein. Using this antibody, we identified the xpac protein in the nucleus of cells. In normal human cells, 40- and 38-kDa proteins were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A reduced amount of the smaller protein was detected in XP 39OSSV cells, which show low UV sensitivity, and no xpac proteins were detected in XP 2OSSV cells, which show high UV sensitivity. These levels of xpac proteins in xeroderma pigmentosum cells were determinants of heterogeneity of the DNA repair defect in group A xeroderma pigmentosum. Synthesis of the xpac protein did not increase after UV irradiation.