p67 expression defines erythroid progenitor cells. (A) p67 expression on CD34⁺ cells: p67 expression was detected on CD34⁺ cells from BM by staining with anti-p67Ab. (Representative FACS histogram, n = 8; grey area indicates FITC-labeled secondary Ab; black line, anti-p67⁺ FITC-labeled secondary Ab.) (B) Differential p67 expression on BM and MPB CD34⁺ cells: 34% ± 4% (mean ± SEM; range: 20%-48%) of BM CD34⁺ cells expressed p67. The frequency of p67-expressing cells among MPB CD34⁺ cells was 50% ± 5% (range: 38%-60%, P = .024, 8 donors for BM, 5 donors for MPB). (C) p67 expression by CD34⁺ cells is selective for erythroid cells: Among MPB CD34⁺ cells, glycophorin A⁺ cells were relatively infrequent (5.1%). However, 99% of the rare CD45^dim glycophorin A⁺ cells (top) and 100% of the CD45⁺ glycophorin A⁺ cells (middle) expressed p67. p67 was also expressed on less than 10% of CD45⁺ glycophorin A^– cells (bottom). Similar data were obtained from 2 samples of BM cells: Although in these BM samples CD34⁺ glycophorin A⁺ cells were even less frequent (0.2%), essentially all coexpressed p67 (not shown). (D) Coexpression of p67 and erythroid lineage markers: The majority of early BFU-E–derived cells coexpress glycophorin A and p67. By morphology and benzidine staining, these cells were identified as proerythroblasts (Figure S1) (representative FACS dot plot from 3 independent experiments). (E) Myeloid cells do not express p67: CFU-GM–derived cells, identified as myeloid cells by CD45 and CD11a expression and negativity for glycophorin A, did not express p67 (representative FACS dot plot from 2 independent experiments).