Proliferation assay in response to hIL-3 (A) or mGM-CSF, as a control for cell growth and viability (B) by FDC-P1 cells expressing either IL-3Rα + hβc (○) or IL-3RΔNT + hβc (□). Cells expressing IL-3RΔNT + hβc show a mean 14-fold decrease in responsiveness to hIL-3 (ED₅₀ = 518 pmol/L) compared with IL-3Rα + hβc (ED₅₀ = 37.4 pmol/L, n = 3, P < .014). Data are the mean of triplicate points ± SD.