Pak2 regulates SDF1α- and integrin-induced filopodia formation in HSPCs in a kinase- and β-Pix–dependent manner. WT and Pak2^fl/fl LSK cells were transduced with indicated retroviruses and FACS sorted. EGFP⁺ cells were stimulated with SDF1α and stained for F-actin using rhodamine-labeled phalloidin and counter-stained with DAPI. The slides were then imaged using Zeiss LSM 700 Laser Scanning confocal microscope with a 100× objective lens. (A) Representative images of Control (WT-Cre) and Pak2^Δ/Δ. (B) Representative images of Pak2^fl/fl cells transduced with Cre coexpressing Pak2-WT, Pak2-KD, and Pak2-Δβ-Pix mutants showing F-actin (red), DAPI (blue), overlap (merge). The boxed region is enlarged in grayscale in the inset to better define filopodia and abnormal protrusions containing F-actin. Red arrows indicate filopodia, white arrows indicate excessive protrusions, and yellow bars indicate abnormal F-actin bundles. The data are from 1 of 2 or more independent experiments each scoring at least 70 cells.