Pak2 is a negative regulator of megakaryocyte endomitosis. (A) Megakaryocyte DNA content in wild-type and Pak2^−/− mouse bone marrow, 14 DPI, was measured in CD41⁺ bone marrow cells by flow cytometry. Line indicates location of 8N+ cells (n > 10; mean ± SEM; *P < .05). (B) Megakaryocyte DNA content in wild-type and Pak2^−/− in vitro-derived megakaryocytes. Bone marrow cultured for 5 days with TPO and 500 nM 4-hydroxytamoxifen to activate Cag-Cre-ERT2;Pak2^fl/fl transgene (n > 4, mean ± SEM; *P < .05). (C) Megakaryocyte DNA content in vehicle and Frax1036-treated mice. Mice were dosed with Frax1036 via oral gavage daily for 21 days. Frax1036 (Frax) ablates Pak1/2/3 Serine 141 phosphorylation (pPak) in the bone marrow relative to total Pak1/2/3 (Pak) (WB inset). Percentage greater than 8N DNA content, mean ± SEM; *P<.05. (D) Percentage of CD41⁺ bone marrow cells with Frax1036 treatment. Four mice/genotype, mean ± SEM; P < .001. (E) Bone marrow-derived megakaryocytes cultured with 0.5 μM Frax1036 for 5 days; n > 5 mice. 8N and 16N populations significantly increased in Frax1036-treated bone marrow (P < .008 and P < .003, respectively).