MicroRNAs (miRNAs) are short (20–23-nucleotide), endogenous, single-stranded RNA molecules that regulate gene expression1 . Mature miRNAs and Argonaute (Ago) proteins form the RNA-induced silencing complex (RISC), a ribonucleoprotein complex mediating posttranscriptional gene silencing2–5. Complementary base-pairing of the miRNA guides RISC to target messenger RNAs, which are degraded, destabili..