Internalization of wt and mutant G-CSF receptors in 32D.cl8.6 cells. (A) 32D cells expressing various mutant G-CSF receptors were allowed to bind biotinylated G-CSF at 4°C and were subsequently incubated at 37°C for 0, 15, or 60 minutes at 37°C before staining with SA-PE to determine surface-bound biotinylated G-CSF. Shown are internalization profiles of a representative 32D clone. Bold line indicates 0 minutes; dotted line, 15 minutes; thin line, 60 minutes; and shaded histogram, 0 minutes, with initial binding in the presence of excess nonbiotinylated G-CSF. (B) Quantification of the mean percentage of internalized biotinylated G-CSF in 32D cells. Shown are mean values and SEMs of the percentage of internalized receptors, determined by assessing peak channel values of fluorescence of at least 3 different clones per construct after 60 minutes of internalization (*P < .05).