Srf and the Mrtfs are required for the chemotactic response to SDF-1. (A) Srf⁺ (mT) and SrfKO^H (mG) fetal liver LSK cells were mixed 1:1, plated onto fibronectin-coated 5-µM pore transwells, and allowed to migrate across the membrane toward SDF-1 or SCF for 4 hours. See supplemental Figure 7B. (B) Srf⁺ or SrfKO^H fetal liver LSK cells prelabeled with CFSE were plated onto the upper chamber of a fibronectin-coated 5-µM pore transwell and allowed to migrate toward SDF-1 for 45 minutes. After fixation, cells were stained with Texas Red phalloidin and imaged by confocal microscopy, and the cells associated with pores in 12 to 13 fields were counted. (C) Srf⁺ or SrfKO^H fetal liver LSK cells were plated and processed as in (B), and Z-stacks acquired. Top panels, proportions of cells at different locations. Black and red arrows, upper and lower membrane surfaces. Bottom, cell numbers: NP, not associated with pores; PA, pore-entrance–associated; TM, transiting pore. (D) RNA-seq analysis of fetal liver LSK cells. Volcano plots of fold-change in RNA expression upon Srf inactivation vs statistical significance (left) and of SDF-1 induced transcripts in Srf⁺ cells. (E) Comparison of positively-regulated Srf-dependent gene sets in LSK and MEFs at significance P < .05. When a fold-change threshold of 2 is set for Srf dependence, 245 genes are positively regulated in LSKs, 657 in MEFs, and only 12 in both cell types. 1951 genes were negatively regulated by Srf in LSKs, of which 155 were shared with MEFs.