The cells in the erythroid series derived from myeloid progenitor cells or from the bi-potential megakaryocyte-erythroid progenitor cells which eventually give rise to mature RED blood cells. The erythroid progenitor cells develop in two phases: erythroid burst-forming units (BFU-E) followed by erythroid colony-forming units (CFU-E); BFU-E differentiate into CFU-E on stimulation by erythropoietin, and then further differentiate into erythroblasts when stimulated by other factors.