Inactivation of both Mrtfs phenocopies SRF deletion. (A) E18.5 femurs stained with H&E (left) or endomucin (right). (B) Gr-1⁺ cellularity in mutant Mrtf mutant E18.5 long bones. Data are from 6 embryos of each genotype (P < .0001; unpaired Studen t test). (C) Colony-formation assays by cells from Mrtf-mutant MrtfabKO^H E18.5 long bones. Data are from 3 embryos of each genotype, and each assay was performed in triplicate. (D) Proportion of LSK cells in Mrtf E14.5–mutant fetal livers. (E) Colony-formation assays with cells from Mrtf-mutant E14.5 fetal livers. Data are from 3 embryos of each genotype, and each assay was performed in triplicate. (F) Mrtf⁺, Mrtfa^−/−, MrtfbKO^H, or MrtfabKO^H fetal liver LSK cells were mixed and plated onto fibronectin-coated 5-µM pore transwells and allowed to migrate across the membrane toward SDF-1 for 4 hours. (G) Roles of MRTF-SRF and chemokine signaling in tissue colonization by HSC/P. See “Discussion” for details.