Expression of a kinase-inactive Lyn mutant increase TPO-induced proliferation. BaF3/Mpl cells were transfected with either dominant-negative Lyn or wild-type Lyn cDNA. (A) Equal cell surface expression of Mpl was determined by flow cytometry using a primary anti-Mpl antibody and a secondary phycoerythrin (PE)–conjugated antirabbit antibody. In the lower panel, a Western blot demonstrates the relative Lyn expression in each cell line. (B) Cell proliferation assays were used to compare BaF3/Mpl cell lines expressing the mutant (K275L) or wild-type forms of Lyn. Data represent the mean and SD of triplicate wells. (C) Lyn immune complex kinase assay was performed as described in Figure 1. Kinase data are representative of 3 independent experiments. Error bars indicate SD.