GPS platelets demonstrate a spreading defect. (A) Fluorescence microscopy of control and GPS platelets spread for the indicated amounts of time on poly-L-lysine and stained with Alexa 546–phalloidin. (B) Quantification of surface area and perimeter of spread platelets from control (solid circles, solid lines) and GPS (open circles, dashed lines) subjects spread on either poly-L-lysine, collagen, or fibronectin as indicated. Error bars represent the standard error of the mean (SEM) of measurements from 150 to 250 platelets per time point.