Induction of tyrosine phosphorylation of platelet proteins after stimulation with TPO. Platelets were unstimulated (-) or stimulated (+) with TPO, 100 ng/mL, followed by lysis and immunoblotting. Platelet proteins (35 μg) were loaded and separated on a 7.5% polyacrylamide gel. After protein transfer, membranes were incubated with an HRP-conjugated antiphosphotyrosine antibody followed by ECL detection. Stimulation of normal platelets (left lanes) with TPO led to phosphorylation of proteins of apparent molecular weight 95 kDa (arrow). Shown are the results of 2 affected family members (middle and right lanes) who fail to reveal TPO-mediated signaling. This experiment was performed in duplicate with similar results.