Chemoresistance genes have been identified as an impediment to anticancer drug treatment. In particular, P-glycoprotein, the product of the multidrug-resistance (MDR1) gene, plays a major role in clinical treatment failure. Conversely, expression of an MDR1 cDNA in bone marrow of transgenic animals renders hematopoietic cells chemoresistant. Efficient transfer of drug-resistance genes to normal hematopoietic progenitor cells has been achieved with the use of retroviral vectors. In this article we review approaches which use the multidrug-resistance gene to protect bone marrow from myelosuppression following chemotherapy and as a selectable markerin vivo to increase the expression of nonselectable genes which correct hereditary diseases of the hematopoietic system.