The main function of red cells is to transport oxygen from the lungs to the peripheral tissues. Erythropoietin controls red-cell production by stimulating the differentiation of erythroid progenitor cells in the bone marrow. The hormone originates mainly in specialized interstitial renal cells, which respond to a decrease in oxygen delivery by increasing their production of erythropoietin. The purification of erythropoietin from the urine of patients with aplastic anemia by Miyake et al.¹ permitted cloning of the erythropoietin gene in 1985. The commercial production of recombinant erythropoietin soon followed, opening the way to the first clinical trials of a hematopoietic growth . . .