Pak2 is required for HSPC engraftment. (A-D) In vitro assays and homing of Pak2 cells. WT and Pak2^fl/fl LSK were transduced with MIEG3 empty vector (MIEG3) or Cre-MIEG3+ (Cre) 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. Cre-transduced Pak2^fl/fl LSK showed aberrant cell morphology compared with Pak2^fl/fl transduced with MIEG3 and WT controls in response to stimulation with SDF-1α. The changes in cell shape were quantified by (A) total cell area (uM²) and (B) cell perimeter (uM). Data represent mean ± SEM (N = 7-19 cells analyzed per group). **P < .01. (C) LSK cells from WT and Pak2 ^fl/fl were transduced with MIEG3-Cre-EGFP virus. The 250 000 sorted GFP⁺ cells were injected into lethally irradiated mice. Homing of the transplanted cells was measured at 12 hours posttransplantation using bone marrow from the femur, tibia, and iliac crest. Pak2 homing efficiency is graphed as a percent of WT-Cre-transduced LSK. (N = 9). **P < .001. (D) 1.0 × 10⁵ Pak2^fl/fl + MIEG3 (black) cells, Pak2^WT/WT +Cre-GFP (gray) cells, or Pak2^fl/fl + Cre-GFP (white) cells were co-transplanted with 5.0 × 10⁵ CD45.1 whole bone marrow into sublethally irradiated NOD-SCID recipient mice. Data represent mean ± SEM (N ≥ 6 recipients per genotype). *P < .05; **P < .01 of 2 independent experiments.