(A) No significant difference is seen in the viability of CD34⁺ cells treated with or without GM-CSF for 72 hours, as measured by colony-forming ability. (B) GM-CSF treatment of CD34⁺ cells reduces expression of HOX A5 mRNA. cDNAs from GM-CSF–treated and untreated populations were amplified with GAPDH- and HOX A5-specific primers. GAPDH products are visualized by ethidium bromide staining, and HOX A5 products are visualized by Southern blotting with a HOX A5-specific³²P-labeled probe. Controls containing mock cDNA reactions are labeled as “−RT.” The diffuse signal below the HOX A5 band is hybridization of the probe to the PCR primers. (C) Titration experiment demonstrating the parallel linear increase in signal relative to the concentration of input mRNA. Semiquantitative PCR was conducted as described in Materials and Methods.