Total RNA was extracted from cultured cells using TRIzol reagent (Invitrogen) according to the manufacturer's instructions. RNA quality and integrity were assessed using an Agilent 2100 Bioanalyzer (RIN value > 8.0). poly(A) mRNA was enriched using oligo(dT) magnetic beads, fragmented, and reverse-transcribed into cDNA. The cDNA was end-repaired, A-tailed, ligated to adapters, and PCR-amplified.