JNK1^−/− mice display features of unstable hemostasis in vivo. The role of JNK1 in hemostasis was investigated in an in vivo tail-bleeding assay. (A) Bleeding times for WT (●) and JNK1^−/− mice (○). Means are indicated by horizontal lines. Statistical significance was determined in 2-tailed Mann-Whitney tests (**P < .01). (B) Percentage of WT (■) and JNK1^−/− (□) mice exhibiting rebleeding. Results were obtained from 33 WT and 30 JNK1^−/− mice. (C) Images of WT and JNK1^−/− platelet ultrastructure, obtained by electron microscopy. Bar represents 1 μm. (D) Size of WT and JNK1^−/− platelets, evaluated by flow cytometry and based on forward scatter (FSC). Data are mean plus or minus SEM (n = 12; unpaired Student t test). Surface expression of integrin αIIbβ3, GPIbα, and GPVI, as determined by flow cytometry with FITC-conjugated anti–mouse CD41 mAb (MWReg30; E), FITC-conjugated anti–mouse GPIbα mAb (Xia.B2; F), or FITC-conjugated anti–mouse GPVI mAb (JAQ1; G). Results are expressed as mean fluorescence intensities (MFI) plus or minus SEM (n = 3; unpaired Student t test).