Impaired Rap1 activation in platelets from JAK2V617F-positive ET patients. Washed platelets from control and ET patients were stimulated for 5 minutes with SFLLRN, TPO, or a combination of TPO and SFLLRN. Rap1 activation was subsequently analyzed by GST-RalGDS pull-down and immunoblotting for Rap1 (i). The bar graph shows the average (± SEM) fold-change in Rap1 activation on SFLLRN stimulation (ii, n = 7).