Platelets from Unc13d^Jinx mice demonstrate a spreading defect. (A) Fluorescence microscopy of platelets from wild-type and Unc13d^Jinx mice spread for the indicated amounts of time on poly-L-lysine and stained with Alexa 546–phalloidin. Scale bars, 5 μm. (B) Quantification of surface area and perimeter in platelets from wild-type (solid circles, solid lines) and Unc13d^Jinx (open circles, dashed lines) mice spread on either poly-L-lysine, collagen, or fibronectin as indicated. Error bars represent the SEM of measurements from 150 to 250 platelets per time point.