Titration of the neutralizing MoAb 7G3 against a fixed dose of hIL-3 (322 pmol/L) on FDC-P1 expressing either IL-3Rα + hβc (○) or IL-3RΔNT + hβc (□), showing complete neutralization of proliferation induced by hIL-3 on cells expressing IL-3Rα + hβc, but no inhibition of IL-3–induced proliferation of cells expressing IL-3RΔNT + hβc. This confirms that the MoAb 7G3 cannot bind to the truncated receptor. Data are the mean of triplicate points ± SD.