PEM population in the hierarchy of human hematopoiesis. The schema indicates the proposed placement of PEM in the erythrocyte-megakaryocyte differentiation arm of human hematopoiesis. The expression patterns of key CD antigens are shown for principal cell populations. Curved arrow, self-renewal capacity; straight arrow, differentiation path; broken arrows, minimal self-renewal or differentiation potential. References for progenitor-specific markers: MEP^33-35,40,41 ; CFU-Meg^{30,33-35,41,42} ; CFU-E/BFU-E.^{33-35,41,43-45}  CD, cluster of differentiation.