CD34⁺ cells stimulate stromal G-CSF mRNA. The indicated numbers of FACS-sorted CD34⁺ cells, CD15⁺/CD14⁻ cells, or both together were plated in transwell inserts above stromal feeders for 24 hours before obtaining stromal cell RNA. (A) Northern analysis, showing increased stromal G-CSF mRNA in the presence of CD34⁺ cells. (B) RT-PCR–amplified products resolved on 1.5% agarose gel, showing a dose-dependent increase in G-CSF mRNA transcripts with increasing numbers of CD34⁺ cells. The positive control was the specified 470 bp cDNA product of human G-CSF obtained from Clontech. The 353-bp amplified product of β-actin is shown as an internal control for RNA.