Srf is critical for neutrophil actin assembly. Srf wild-type (WT) and knockout (KO) neutrophils were allowed to attach to poly-l-lysine–coated coverslips and stimulated with N-formyl-methionyl-leucyl-phenylalanine for 2 minutes. F-actin was stained with phalloidin (red) in Cre-expressing neutrophils marked by yellow fluorescent protein; a merge with 4,6 diamidino-2-phenylindole is shown. Note the lack of polarization with decreased F-actin polymerization in Srf-null neutrophils compared with WT cells when activated with N-formyl-methionyl-leucyl-phenylalanine. See Figure 4A in the article by Taylor et al that begins on page 3027.