Differential response of Id-1 and Id-2 to cytokine stimulation in vitro. FDCP1 cells were starved of cytokines for 24 hours, then transferred into fresh medium (□) or medium containing 100 U/mL of either rIL-3 (♦) or rGM-CSF (▪) at time zero. The cells were allowed to grow for 24 hours, during which time samples were removed for RNA preparation and northern hybridization to (A) Id-1 and (B) Id-2 probes. Signal intensity was quantitated by phosphorimage analysis before the blots were normalized by rehybridization to GAPDH. Values were plotted as the relative intensity (pixel number) of the specific bands above background for each probe. Values are comparable only within the group of samples hybridized to a single probe; different exposure times were used for each probe to maximize clarity of the phosphorimage. (C) Raw data.