Quantitative RT-PCR for miRNAs was performed as previously described . After RNA isolation with TRIzol reagent (Invitrogen #15596), genomic contaminants were removed using DNAse (Life Technologies #AM1907). DNA-free RNA was then polyadenylated (New England Biolabs #M0276) and subsequently reverse transcribed (Invitrogen #18080) using a pool of specially designed primers at a concentration of 50 uM to generate cDNA copies of polyadenylated miRNAs. A complete list of primer sequences appears in Supplementary Table 3. All subsequent steps of the cDNA synthesis were conducted according to kit directions.