CD9 ligation induces tyrosine phosphorylation of the tyrosine kinases Fgr and Hck. Purified monocytes were either unstimulated or stimulated with the appropriate mAb. (A) Cells were lysed and subjected to SDS-PAGE before transferring to nitrocellulose membranes and immunoblotting with antiphosphotyrosine mAb. The same quantity of protein was loaded in each well. Molecular mass markers are indicated. (B-E) Cells were lysed and immunoprecipitates prepared using either an anti-Hck Ab (B, D) or an anti-Fgr Ab (C, E). After blotting, filters were probed with an antiphosphotyrosine mAb (B-C). The filter shown in B was stripped and reprobed with an anti-Hck Ab revealing identical amounts of Hck in all samples (D). The filter shown in C was stripped and reprobed with an anti-Fgr Ab revealing identical amounts of Fgr in all samples (E). Arrows indicate phosphorylated substrates; NS, no stimulation.