This research proposal involves the development of a mammalian vector system that replicates episomally and the development of new methods for the insertion of cDNA libraries into it. Because the vector remains episomal, it can be recovered rapidly and readily. In this respect it differs from previously described vectors that integrate rapidly into the host (human) cell genome, and hence, cannot be recovered readily. This system can be used to complement defective functions of human cells where the gene is unknown and where a selection can be imparted. Part of the proposal is to continue development of the vector system. Additionally the investigators propose to identify human genes which have not previously been identified: 1) Ataxia telangiectasia, specifically complementation group D; 2) gene(s) coding for methotrexate transport, genes which in a defective form result in a common form of methotrexate resistance in cancer cells.