It has been shown that the elderly have significantly lower numbers of bone marrow early erythroid-committed progenitors (BFU-E) compared to younger subjects, however comparison of anemic and non-anemic elderly populations demonstrates no difference in committed progenitor cell numbers or the effects of EPO on colony-forming unit erythroid (CFU-E) formation.[22] Moreover, our analysis demonstrates that erythropoietin levels are decreased in patients with anemia of unknown etiology, suggesting that the main cause is erythropoietin underproduction rather than decreased marrow reserve. Here, EPO is linked to anemia (phenotype).