Induction of tyrosine phosphorylation of β_IL3 is dependent on the membrane-proximal EpoR cytoplasmic region involved in the activation of Jak2. A clone of 32D cells expressing the S-mutant EpoR with carboxy-terminal deletion of 145 amino acids (A) or the PM4-mutant EpoR, which harbors a mutation, Trp²⁸² to Arg, in the membrane-proximal cytoplasmic region that abolishes the ability to couple with Jak2 (B) were examined as described in the legend to Fig 1.