The level of c-kit mRNA (•) but not that of the GM-CSF receptor β-chain mRNA (○) is downregulated by stroma interaction. Semiquantitative PCR was performed from TF-1 grown for different times on MS-5 stroma. Actin was used as an internal standard for the quantification. The relation of receptor RNAs to β-actin RNA in TF-1 cells grown in suspension were set to 100.