Comparison of phenotypes between human WAS, mouse WASp^−/−, and mouse WIP^−/− platelets. (1) WAS platelet volume is also reduced. (2) Anti-GPVI antibody JAQ1 induces shedding of surface GPVI in wild-type mouse platelets, and partially in those WIP^−/− mice that do not have surface IgA bound. However, the majority (76%) of WIP^−/− mice show IgA bound to the platelet surface, and this correlates with complete protection from GPVI shedding. (3) WAS patients show a variety of mutations in the WASp gene. Coding mutations most commonly arise in the N-terminal WH1 domain. (4) Responses measured included shape change, actin assembly, Arp2/3 activation, PLCγ2 phosphorylation, cytosolic calcium rise, α-granule secretion, and αIIbβ3 activation. ND indicates not determined.