GPIb-dependent FeCl₃-induced stable occlusion of the carotid artery requires Btk. (A) The tracings are representative of blood flow in FeCl₃-treated exposed carotid arteries of wild-type and Btk^xid/Y mice. Blood flow was monitored for 45 minutes, including a 3-minute treatment starting at zero time with a strip of filter paper soaked in 10% FeCl₃. The arrow indicates the time of removal of the FeCl₃-containing strip of filter paper. (B) Box-plot graphs represent time of first occlusion of carotid arteries (blood flow less than 0.2 mL/min) from 10 WT mice and 6 Btk^xid/Y mice. The results show that the median time required for the first thrombotic occlusion in 10 Btk⁺/Y mice was 4.5 minutes, with a range of 2 to 8 minutes; the median time required for the first thrombotic occlusion in the Btk^xid/Y arteries was 16 minutes, with a range of 6 to 45 minutes. Arterial blood flow was occluded in all the wild-type arteries at the end of the monitoring period. On the contrary, all of the treated Btk^xid/Y arteries were patent at the end of the monitoring period. No occlusion occurred in the treated carotid artery of one Btk^xid/Y mouse. These results demonstrate that Btk is required for GPIb-dependent stable thrombus formation in vivo. Bars represent extreme values for times of first occlusion.