Analysis of _IIb and β₃proteins in Cos-7 cells by immunoblotting. Fifty micrograms of proteins from each cell lysate transiently cotransfected with wild-type β₃ and wild-type or mutated _IIb as indicated were separated on 7% polyacrylamide gels without disulfide reduction and transferred to nitrocellulose membrane. One microgram of protein from normal platelets was used as a positive control; mock refers to cells transfected with expression vector alone. An MoAb specific for _IIb (SZ22) and an MoAb specific for β₃ (XIIF9) were used.