Extrinsic noise vs. intrinsic noise-induced bimodality in post-transcriptional regulation experimentsMicroRNAs (miRNAs) are a class of nearly 22 nucleotide-long, non-coding RNA molecules involved in the post-transcriptional regulation of their target RNAs. MiRNAs and their targets interact via a titration-like mechanism characterized by threshold effects and cross-talk among targets. Threshold has to be understood as a value of mRNA transcription rate such that above this value many target molecules are available for translation while below it mRNA are bound to miRNA, rapidly degraded and cannot be translated. It has been discovered that different RNA species may compete for miRNAs binding with this inducing indirect interactions amongst the miRNA targets which reciprocally influence their expression levels.