The EpoR extracellular and transmembrane domains are not required for the induction of β_IL3 tyrosine phosphorylation. A clone of BaF3 cells expressing the wild-type EpoR (BaF3/EpoR-Wt) or a chimeric receptor in which the extracellular and transmembrane domains of the EpoR were replaced with those of the EGFR (BaF3/EGF-EpoR) was starved for 12 hours and left unstimulated (C) or stimulated with 100 U/mL of Epo, 25 ng/mL of IL-3, or 100 ng/mL of EGF, as indicated, for 1 minute. The cells were lysed and subjected to immunoprecipitation with anti-β_IL3 , anti-EpoR, or anti-EGFR, as indicated, followed by antiphosphotyrosine immunoblotting (αPY). In (A), the membranes were stripped and reprobed with anti-β_IL3 , as indicated. The size markers are indicated and given in kD.