A naturally occurring hormone that is produced in the bone marrow, erythropoietin stimulates the production of red blood cells, and recombinant human erythropoietin (EPO) and follow-ons such as Thousand Oaks, Calif.-based Amgen Inc.’s Epogen (epoetin alfa) and Aranesp (darbepoetin alfa) are a billion-dollar industry for the treatment of anemia.