Synthetic interaction of megakaryocyte GATA-1 deficiency with P-TEFb inhibition. (A) GATA-1Lo mice (G1Lo) and age-matched wild-type C57BL/6 received 5 mg/kg per day of flavopiridol (FP; 5 mice per group) or saline control (4 mice per group) for 7 consecutive days. Blood samples from the retro-orbital plexus were then analyzed on a Hemavet automated analyzer for platelet counts. (B) Mice, as in panel A, received a 9-day course of flavopiridol (FP) at the same dose or saline. Marrows were then analyzed by FACS. In rows 1 and 2, marrow cells were costained with anti-CD41 and propidium iodide (PI). The CD41⁺ cells (percentages indicated in row 2) were gated for ploidy analysis using FlowJo software. Numbers in row 1 indicate percentage of total cells within each ploidy category. In rows 3 and 4, marrow cells were costained antibodies to CD41, TER119 (erythroid marker), and CD71 (transferrin receptor), with quadrant percentages as indicated.