Integrin or chemokine receptor engagement on human NK cells results in Cdc42 activation. Human NK cells were left untreated (−) or stimulated with GAM cross-linked anti-α4 (HP2/1), anti-β1 (TS2/16), anti-β2 (TS1/18), or anti-CD56 (C218) mAb (A) or with VCAM-1–, ICAM-1– or BSA-coated beads (B) or with CXCL12/SDF-1 or CX3CL1/fractalkine (C) for the indicated time periods at 37°C. Cell lysates were incubated with GST-PAK fusion protein, and bound active GTP-Cdc42 molecules were evaluated by Western blotting with anti-Cdc42 mAb (top). Cell lysates probed for total Cdc42 are shown as loading control (bottom). Sizes are indicated in kilodaltons. These results represent 1 of 3 independent experiments.