Immunoprecipitation analysis of biotin surface-labeled platelets derived from a Sit^a (−) and a Sit^a(+) individual with anti-Sit^a (lanes 1 and 4), anti-Br^a (lanes 2 and 5), and anti-Br^b (lanes 3 and 6). Immunoprecipitates were separated on 7.5% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreduced conditions, transferred onto nitrocellulose membrane, and visualized using streptavidin-horseradish peroxidase and chemiluminescent substrate.