Cdk9 inhibitor interferes with primary human megakaryocyte development. (A,B) Purified human CD34⁺ cells underwent culture 5 days in unilineage megakaryocytic medium containing TPO, SCF, stromal-derived factor-α, and the indicated doses of flavopiridol (FP). (A) Flow cytometry (FACS) assessment of CD41 expression and forward light scatter (FSC), a reflection of cell size. Analyses were performed on gated viable populations, with percentages of CD41^bright FSC^hi (mature megakaryocytes) and CD41^bright FSC^lo (promegakaryocytes) cells determined using FlowJo software. Wright-stained cytospins were photographed (original magnification ×100). (Top row) Two fields with typical polyploid control megakaryocytes (). (B) Ploidy analysis of CD41⁺ and CD41⁻ cells within the cultures. Cells were stained with FITC-anti-CD41 and propidium iodide, followed by FACS analysis and quantitation with FlowJo software.