Identification of mutant GPIIb in cells expressing del.20 and alt.25. Shown are Western blots of immunoprecipitated GPIIb and GPIIIa from lysates of cells transfected with normal GPIIIa and normal or mutant GPIIb cDNAs. (A) Immunoprecipitation by polyclonal antibody against GPIIb and detection of GPIIb by SZ22 monoclonal antibody revealing both mature GPIIb and pro-GPIIb in WT cells, only pro-GPIIb in GPIIb-alt.25, and a fast-migrating pro-GPIIb in GPIIb-del.20. (B) Immunoprecipitation with CA3 monoclonal antibody against GPIIb followed by Western blot analysis with SZ22 revealing the same pattern as in panel A but an additional smaller band in the mutants probably representing a GPIIb degradation product.