Effect of V449E cytoplasmic truncations on factor-independent growth and viability. (A) Cell surface expression of the indicated FLAG-tagged hβc proteins on FDB1 cells expressing cytoplasmic truncations of the V449E activated mutant. Infected cells (solid lines) were stained with an anti-FLAG monoclonal antibody and compared with identically stained uninfected cells (dotted lines). (B) Proliferation of FDB1 cells expressing cytoplasmic truncations of the V449E activated mutant. Growth without factor (NF indicates no 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 indicate the SD of triplicate samples. (C) 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.