It is also expressed in hematopoietic progenitor cells including erythroblasts, myeloblasts, and megakaryocytes. Circulating erythroblasts and erythroblasts in the bone marrow show various morphologic abnormalities. The protein is synthesized by erythroblasts and secreted. Transfusion of in vitro-generated RBCs or erythroblasts does not sufficiently protect against tumor formation. Cells which synthesize heme are predominantly erythroblasts/reticulocytes in the bone marrow (80%) and hepatocytes (20%).