Fig. 2. Growth inhibition by IL-9 is correlated to the level of expression of the transfected receptor. A, IL-9R expression was measured in 11 BW5147 clones by flow cytometry with an anti-IL-9R antibody (6) . Growth inhibition by hIL-9 was measured as described in Fig. 1 and presented as a ratio between hexosaminidase levels in the absence and presence of IL-9. Linear correlation was calculated using InStat software. B, untransfected cells, clone 6, and clone 45 were stained with a hybrid protein consisting of a constant immunoglobulin 3 domain and either human (solid line) or murine (dashed line) IL-9 and with antimouse immunoglobulin antibodies coupled to FITC. Cells were analyzed by flow cytometry.