Type IIB von Willebrand factor with normal sialic acid content induces platelet aggregation in the absence of ristocetin. Role of, platelet activating, fibrinogen and two distinct membrane receptors - DeMarco, L; Mazzuccato, M; Del Ben, M G; Budde, V; Federici, A B; Girolami, A; Ruggeri, Z M (J Clin Invest, 1987)