Srf KO neutrophils lack polarization with decreased f-actin polymerization when activated with fMLP.Srf WT and KO neutrophils were allowed to attach to poly-l-lysine–coated coverslips and stimulated with fMLP for 2 minutes. (A) F-actin was stained with phalloidin (middle) in Cre-expressing neutrophils marked by YFP (bottom); merge with DAPI is shown in the top panels. Quantification of polarized cells from 3 independent experiments; Srf WT, n = 71; Srf KO, n = 91 (B). G-actin (C) and F-actin (D) were assessed at 0, 30, 60, and 90 seconds after stimulation with fMLP by staining with DNAse I (G-actin) and phalloidin (F-actin) and mean fluorescence intensity (MFI) determined by flow cytometry. Representative of 3 independent experiments performed in triplicate.