Inhibition of PAK in HSPC leads to excessive cell spreading and protrusions in response to SDF-1 chemokine stimulation. WT LSK was transduced with PID or MIEG3 retrovirus. GFP+ LSK were isolated 48 hours later using fluorescence-activated cell sorter, and were stimulated with SDF-1α and plated on fibronectin-coated coverslips. (A) PID transduced LSK showed aberrant cell morphology compared with MIEG3 controls in response to stimulation with SDF-1α for 5 minutes and staining of F-actin with rhodamine-labeled phalloidin (red) and DAPI-stained nuclei (blue). The changes in cell shape were quantified by (B) total cell area (uM²) and (C) cell perimeter (uM). Data represent mean ± SEM (N ≥ 40 cells analyzed per group). **P < .001.