Mk/Ery specification is skewed in the absence of RUNX1C. (A) Schematic representations of the Runx1 WT (top) and P1-MRIPV (bottom) alleles. White bars, noncoding regions; black bars, common coding regions; colored (red or blue) bars, unique coding regions. mRNA, messenger RNA. (B-C) Peripheral blood parameters of WT, P1-MRIPV/⁺, and P1-MRIPV/MRIPV mice as determined by Sysmex automated cell counting. (B) Red blood cell, white blood cell, and platelet counts; (C) plateletcrit. (WT, N = 5; P1-MRIPV/⁺ and P1-MRIPV/MRIPV, N = 6). (D-F) Flow cytometric analysis of WT, P1-MRIPV/⁺, and P1-MRIPV/MRIPV BM Mk/Ery progenitor (PreMegE, MkP, PreCFUe, and CFUe) and LSK HSPC populations. (D) Representative FACS plots of Mk/Ery progenitors; (E) representative FACS plots of LSK HSPCs; (F) proportions of HSPCs as a percentage of total live BM cells; N = 6. *P < .05; **P < .01; ***P < .001; ****P < .0001.