Recent advances in molecular genetics have made possible the use of retroviral "vectors" to transfer cloned human genes into somatic cells. With this new technology, the genetic defect underlying many recessive inherited disorders can probably be corrected by inserting a normal gene into the patient's hematopoietic stem cells. This article reviews the design and safety of the viral vectors and the results of in vivo studies in mice and large animals that have led to the first human trials. Other target cells for gene transfer, such as endothelial cells, fibroblasts, keratinocytes, and hepatocytes, are also discussed. The use of recombinant retroviruses for gene transfer in vivo is still a new area of research, but the feasibility of "gene therapy" for genetic disorders is rapidly gaining medical and scientific acceptance.