Recognition of substitution mutations in the N-terminus of the murine GPIIIa by the monoclonal antibody, SZ21. The same modified murine proteins assayed in Fig 2 (samples a-k) were tested for immunoreactivity with the monoclonal antibody, SZ21. The immunoblot with SZ21 (A) can be compared with that with a clinical anti–HPA-1a antibody (B) to illustrate the difference in binding to the position 39-containing mutations. (C) Coomassie blue–stained gel of the proteins. Approximate molecular weight of the protein of interest is indicated on the right. The contaminating GST protein (lower band) is present on the stained gel.