Western blot analysis of plasma GPIbα. Platelet-poor plasma was immunoprecipitated with 142.2, analyzed by SDS-PAGE on an 8% to 16% exponential gradient in the presence of β-mercaptoethanol, and immunoblotted with MBC 142.6. Plasma samples from the parents and the normal volunteer were diluted 1:2. Each patient's plasma contains a soluble GPIbα, similar to glycocalicin, although there is significantly less in the two patients as compared with the parents and normal sample.