Effect of GM-CSF deprivation on expression of MDM2 mRNA in TF-1 AML cells. TF-1 AML cells (3 × 10⁵ cells/mL) were cultured for 2 days in the presence of GM-CSF (1 ng/mL), washed three times with PBS, and resuspended in fresh media without GM-CSF. Viable cells obtained by Ficoll density gradient sedimentation were prepared at each time point (0, 2, 4, 8, and 16 hours), and 10 μg of RNA from each sample were separated by Northern blotting and hybridized with an MDM2 specific cDNA probe. Rehybridization with β-actin confirmed equal mRNA loading.