Patients with PTPRJ-null variants present platelets with small size and defective platelet response to convulxin and thrombin receptor activating peptide (TRAP). (A) Peripheral blood smears, May-Grünwald-Giemsa staining: representative examples of small-sized platelets from proband I-2 (i-iii) and proband II-2 (iv-v). The images are representative of the average platelet observable in patients’ blood smears. In (vi), an average platelet of normal size for a healthy subject is shown for comparison. Scale bars, 5 µm. (B) Flow cytometry of platelet activation in response to ADP, TRAP, and convulxin (CVX) in probands II-1 and II-2. Platelet surface expression of P-selectin and of the activated form of GPIIb-IIIa (PAC1 antibody binding) was measured after incubation with ADP (5 mM), TRAP (25 µM), CVX (100 ng/mL), or vehicle (HEPES buffer). Platelet activation is expressed as the ratio between mean fluoresce intensity (MFI) measured after stimulation with each agonist and MFI measured after incubation with the buffer alone (resting platelets). The values obtained in the 2 probands were aggregated and compared with those of 3 HCs processed in parallel. Data represent the mean ± SD of 2 independent analyses. *P < .05, **P < .01, 2-tailed Student t test.