Normal actin polymerization of WAS MKs after stimulation by SDF-1 and thrombin. After 7 to 13 days, cultured cells were resuspended and stimulated with thrombin (0.1 IU/mL) or SDF-1 (300 ng/mL) for 30 seconds with shaking at 37°C. Cells were then fixed, permeabilized, and incubated with PE-labeled anti-CD41a and FITC-labeled phalloidin. Cell samples were analyzed on a FacSort (Becton Dickinson). MKs were selected by gating the CD41⁺cells (A). Analysis of FITC intensity on CD41⁺ cells showed that the baseline content of F-actin and the increase of actin polymerization after stimulation of MKs by SDF-1 (B and D) and thrombin (C and E) were the same in normal MKs (B and C) and in WAS MKs (D and E).