Effects of V449E cytoplasmic tyrosine mutants on factor-independent growth and viability. (A) Growth without factor (diamonds) is shown compared with growth with the control cytokine IL-3 (squares). Results representative of at least 3 independent experiments are shown. Error bars represent the SD of triplicate samples. (B) Viability of cells without factor at day 5 after factor withdrawal. Cell viability was determined by trypan blue exclusion, and the combined results of at least 3 independent experiments are shown. Error bars represent the SD of at least 3 independent experiments. (C) Constitutive activation of ERK1/2. Lysates were made from FDB1 cells expressing mutant receptors and were blotted for phospho-ERK1/2 and total ERK1/2 as described in “Materials and methods.”