Reverse transcription was performed at 42°C for 60 minutes with the final denaturation step at 90°C for 5 minutes in 30 µl containing 0.5 µg of total RNA. dNTPs, oligo(dT)15 primer, recombinant RNasin Ribonuclease Inhibitor, and M-MLV Reverse Transcriptase (all from Promega, Madison, WI) were used according to the manufacturer's instruction.