Platelet counts, BM progenitor, and Mk ploidy analysis. Blood from wild-type (WT; n = 15), Pf4/Srf^F/+ HET (n = 12), and Pf4/Srf^F/F knockout (KO; n = 17) mice was analyzed using a Hemavet analyzer. (A) Platelet counts in Pf4/Srf^F/F mice were significantly lower than in control mice (P < .001). (B-C) BM from Pf4/Srf WT (n = 8) and KO (n = 11) mice was analyzed by flow cytometry for CD41⁺ megakaryocytes (B, *P < .001) and progenitor populations (C, **P < .05). (D) BM from WT (n = 3) and KO (n = 4) mice was assayed for CFU-Mk. (E) Plasma TPO levels (in picograms per milliliter) were assayed from Pf4/Srf WT (n = 15) and KO (n = 10) mice. (F) Megakaryocytes from WT (n = 4) and KO (n = 5) mice were analyzed by flow cytometry for nuclear DNA content. *P < .05, **P < .01.