DREAM is required for platelet thrombus formation and hemostasis at the site of vascular injury in mice. Fluorescence intravital microscopy was performed as described in “Methods.” Following laser-induced injury of cremaster muscle arterioles, platelet accumulation was visualized by infusion of DyLight 649–labeled anti-mouse CD42c antibodies into WT or DREAM KO mice. (A) Representative images are shown over the course of 240 seconds after injury. Arrows show direction of blood flow. (B) The integrated median fluorescence intensities of anti-CD42c antibodies (F platelet) were obtained from 42 to 44 thrombi in 6 WT or 6 KO mice and are presented as a function of time. (C) Fluorescence intensities of anti-CD42c antibodies in WT and DREAM KO mice were compared at 30, 90, and 180 seconds after arteriolar injury. (D) Tail bleeding time was monitored by cutting 5 mm of the tail of WT (●) and DREAM KO (▪) mice (n = 13 mice). (E) Blood loss during the bleeding time assay was determined by measuring the absorbance at 575 nm of hemoglobin (Hb). **P < .01 and ***P < .001 vs WT mice after the Mann-Whitney U test. Horizontal bars represent the median value of the fluorescence intensities of anti-CD42c antibodies, bleeding times, or Hb content.