GP6 and Bruton-type agammaglobulinemia: As shown in Btk-deficient human platelets from patients with XLA and Btk-deficient mouse platelets, Btk is required for platelet activation only after low-degree GPVI activation [35,36], whereas Tec compensates for the absence of Btk in GPVI signaling and restores platelet reactivity to high concentrations of collagen- related peptide (CRP) or collagen [35].