Use of the dual CFU-Mk/E assay to quantify bipotent MEPs. (A) Schematic of the dual CFU-Mk/E assay to determine Mk/E potential of FACS-sorted populations. Candidate MEPs are plated in a collagen-based colony assay with EPO, TPO, SCF, IL-3, and IL-6 to promote growth of megakaryocyte and erythroid colonies. After 13 to 15 days of culture, colonies are fixed and stained for CD41a (Mk specific) and GlyA (E specific). (B) Representative images of different colony types are shown. (i) Typical BFU-E stains for GlyA (purple) only. (ii) CFU-Mk stains for CD41a (red) only. (iii-iv) Two representative CFU-Mk/E colonies show prominent CD41a⁺ Mk within a burst of GlyA⁺ cells. Scale bar, 100 µm.