Filamin A binding is unaffected by mutating several hydrophilic residues in GpIbα's large filamin A–binding domain. (A) Mutations used to try to disrupt filamin A contact points without major disruption of the secondary structure of the tail of GpIbα. (B) Such mutations have no affect on the endogenous interaction between recombinant human GpIbα and recombinant human filamin A in CHO cells.