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We have applied an assay based on a monoclonal antibody that discriminates the activity of human DNA polymerase a in rodent-human somatic cell hybrid clones to identify a single genetic locus that is both necessary and sufficient for the expression of DNA polymerase a. We have mapped this locus to the short arm of the human X chromosome, near the junction of bands Xp21.3 and Xp22.1, and demonstrated that it is not expressed from an inactive X chromosome.We have described (1) a panel of monoclonal antibodies specific for DNA polymerase a and have used them for immunoaffinity purification of a complex, presumptively native, form of the enzyme from KB cells that expresses both DNA polymerase a and a tightly coupled DNA primase activity (2, 3) and contains a number of polypeptide components. In the present studies, these antibodies form the basis ofa discriminating immunoassay with which we have mapped to the short arm of the human X chromosome a single genetic locus that is necessary and sufficient for DNA polymerase a expression.

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