Effective retroviral vectors carrying the human HPRT and ADA genes have been described. Initial characterization of the retroviral gene transfer system using the HPRT vector allowed the delineation of several parameters important in viral titer, expression, and stability. Using the HPRT and ADA vectors, we have initiated experiments designed to insert these human genes into various tissues of the mouse and have demonstrated expression of both transduced genes in mouse bone marrow cells. Further work with these and other vector constructions is underway in the hope that this technique may allow safe and effective treatment of ADA and HPRT deficiencies, paving the way for treatments of other inborn errors of metabolism through somatic gene replacement therapy.