Lyn induces tyrosine phosphorylation of the EpoR in COS7 cells. In COS7 cells, the EpoR was transiently coexpressed with Lyn or Jak2, as indicated. Transfected cells were either stimulated with 100 U/mL Epo for 5 minutes or left unstimulated, as indicated (Epo st. + or −, respectively). Cells were lysed and immunoprecipitated with anti-EpoR. Immunoprecipitates were analyzed by antiphosphotyrosine (αPY) immunoblotting followed by reprobing with anti-EpoR, as indicated. A coimmunoprecipitated phosphotyrosyl protein that corresponds in size to Lyn is indicated with an asterisk.