We have mapped murine DNA repair genes to chromosome 4 by the somatic cell genetic approach. We will further study the nature of DNA repair activity which is reexpressed in XP-A cells when murine chromosome 4 is introduced, and do regional mapping of the murine loci complementing XP-A. We will also map XP-B, F, and G genes in the mouse. The main focus of our next year's project is cloning the wild type gene of XP and the human HGPRT gene.