The strategy for cloning a PCR-amplified STAT5 fragment into an expression vector is described. By optimising the magnesium level and the annealing temperature in the PCR reaction, target fragment amplification was achieved, using mouse STAT5b cDNA as a template. The PCR product was cloned and probe identity was confirmed by restriction analysis and sequencing. This probe was used in a solution hybridisation-Rnase protection assay to quantify STAT5 mRNA in female rats' livers and thymus lymphocytes. A higher STAT5 mRNA expression was found in liver.