The cytoplasmic domain of the integrin β3 subunit is required for ligand binding–induced phosphorylation of FcγRIIa. Washed platelets from a patient expressing a homozygous mutant form of a Δ724 β3 integrin subunit were plated onto immobilized fibrinogen or BSA and allowed to spread for 45 minutes at 37°C. Note the failure of platelets harboring a truncated β3 cytoplasmic tail to support tyrosine phosphorylation of FcγRIIa (A) or to spread on immobilized fibrinogen (B,C). Representative pictures are shown in panel B, with the average pixel area of 148 to 350 platelets per condition quantitated and shown in panel C. Error bars show mean plus or minus SEM.