Macrophage progenitors derived from MIEG3-, WT-, E76K-, D61V-, and D61Y-transduced cells express similar levels of the GM-CSF and M-CSF receptors. Given the variable response observed in macrophage proliferation in response to GM-CSF and M-CSF, receptor levels for each of these cytokines was examined. (A) Protein extracts prepared from macrophage progenitor cultures were evaluated for the unique α chain of the GM-CSF receptor. (B) Macrophage progenitors were collected, incubated with anti–M-CSFR, and secondarily stained with PE-conjugated F(ab′)₂ fragment goat anti–rat IgG followed by FACS analysis to determine the percentage of M-CSFR⁺ cells.