c-kit receptor protein is also downregulated by stroma cells, indicating that translational mechanisms are not involved in downregulation and upregulation. Quantification of c-kit protein expression by binding of monoclonal anti–c-kit antibody (YB5.B8) and FACS analysis. Mean fluorescence was compared with the mean fluorescence of TF-1 cells grown in suspension with GM-CSF, which was set to 100%. Unspecific binding as measured by the mouse IgG standard control antibody was gated out. The fluorescence of the stroma-dependent clones was in the range of the control value without c-kit antibody and thus the values were considered not significant (n.s.). Results of at least three independent experiments are shown.