Fig. 4. Transcription of the 5 gene is selectively increased by PRL-induced signals. The levels of 5 transcripts were studied in both BaF-3 and W53 cel lines (grown either in IL-3 or in 30 ng/ml of PRL). Total mRNA was extracted and retrotranscribed to cDNA, and a quantitative PCR was performed to amplify a 630-bp fragment of the 5 mRNA sequence, as described (34) . A control PCR for ß-actin amplification was done to estimate the amount of cDNA in each sample. PCR products were separated by electrophoresis and transferred to Zeta-probe membranes. Signals obtained after hybridization with specific probes were quantified by densitomentry, and the ratio between 5 and ß-actin signals was calculated (bottom column graph). The later data represent means of four different experiments and are shown in a logarithmic scale; bars, SD. The 70Z/3 pre-B cell line was used as a positive control of the amplification reaction.