Platelets from JAK2V617F-positive and negative ET patients demonstrate impaired SFLLRN and TPO-mediated fibrinogen binding. Washed platelets from control subjects, JAK2V617F-positive ET patients (A-H) and JAK2V617F negative ET patients (I-J) were incubated with TPO for 5 minutes (E), JAK2i IV for 15 minutes (H), and stimulated with the indicated concentrations of SFLLRN (A,B,D,E,H,I,J) for 15 minutes in the presence of FITC-fibrinogen (A, n = 14, E, n = 7, H, n = 3, I, n = 6), PE-anti–P-selectin Ab (B, n = 13; J, n = 5), PE-anti-PAR-1 (C, n = 10), PE-anti-integrin α_IIb (D, n = 10) or PE-c-Mpl (G, n = 8). Samples were fixed in 1% formaldehyde and analyzed by FACS analysis. Data are expressed in arbitrary units (MFI, average ± SEM). Alternatively, platelets were stimulated with TPO for 5 minutes, and JAK2 and STAT5 immunoprecipitates immunoblotted with the indicated antibodies (F). The bar graphs show the fold increase in phosphorylation on TPO stimulation (average ± SEM, n = 6).