Effect of RUNX1 mutations on MK differentiation and proplatelet formation. (A-B) CD34⁺ cells from AII-1, AII-2, BII-2, and BII-3 patients and control individuals were grown in liquid medium supplemented with TPO, IL-3, IL-6, SCF, and FLT3-L. C1, C2, and C3 were grown simultaneously with AII-1 and AII-2, and C was grown simultaneously with BII-2 and BII-3. CD41⁺CD42⁺ cells were sorted at day 10 of culture and seeded at 2 × 10³ cells/well in 96-well plate. The percentage of proplatelet forming MKs was estimated at day 13. (A) Representative microscopic images representing control and patients proplatelet forming MKs. (B) The percentage of proplatelet forming MKs was estimated by counting MKs exhibiting 1 or more cytoplasmic processes with areas of constriction. A total of 500 cells per well was counted. The histograms show 1 representative experiment of 3, each in triplicate. Error bars represent ± SD of triplicate (n = 3, P < .05). (C) Ultrastructural aspect of blood platelets from AII-1 and BII-2 patients. Gallery of photographs illustrating the typical platelet abnormalities: large heterogeneity in size with both round enlarged and small thin platelets, presence of giant granules and large vacuoles, presence of incompletely fragmented proplatelets. Similar results were obtained in 3 repeated experiments.