Megakaryocytes (MKs) of the probands exhibit normal in vitro differentiation and defective terminal maturation. MKs were differentiated from peripheral blood progenitor cells through 14-day culture. Samples from probands II-1 and II-2 were processed in parallel with those of 3 HCs. (A) Representative images of MKs labeled with an anti–β1-tubulin antibody (red fluorescence). Hoechst (blue) was used for counterstaining nuclei. Scale bars, 25 µm. (B) MK differentiation was assessed as the yield (absolute number) of CD41⁺ cells at day 14 of culture, as measured by flow cytometry. (C) MK maturation was assessed as the percentage of CD41⁺ cells coexpressing the CD42b antigen, as measured by flow cytometry. *P < .001, 2-tailed Student t test.