Agonist-induced surface expression of the α granule membrane protein CD62p is impaired in HPS model platelets. Washed platelets (8 × 10⁷) isolated from WT, pallid, pearl, or light ear mice were stimulated with the indicated concentrations of thrombin (A and C) or convulxin (B and D) for 10 minutes in the absence or presence of 10 μM ADP as indicated, and then analyzed by flow cytometry for CD62p surface expression. Shown is the percentage of cells with labeling above the background level observed on unstimulated cells (see supplemental Figures 1 and 2 for gating strategies and examples of flow cytometry profiles.). Data represent mean ± standard deviation from at least 3 experiments. *P < .05; **P < .01; ***P < .005 for HPS models vs WT. ns., nonsignificant.