Alleged targets of erythropoietin. The primary task of the hormone (and its therapeutic analogs) is to maintain myeloid erythropoiesis, by preventing erythrocytic progenitors from undergoing apoptosis and by stimulating their proliferation and differentiation. Erythropoietin receptors have been claimed to be present throughout the body, suggesting that erythropoietin is a pleiotropic survival and growth factor. As a result, the alleged erythropoietin's tissue protective potential has already been investigated in clinical trials, for example, in brain and heart diseases. Whether erythropoietin can stimulate cancer growth by promoting tumor cell survival and angiogenesis is an even more controversial issue. Professional illustration by Kenneth X. Probst.