MicroRNA rules: Made to be broken

MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression. For over a decade the deluge of research describing the biogenesis and activity of miRNAs has lead researchers to postulate rules to help make sense of the enormous amount of data produced. These rules are repeated in miRNA research papers and reviews. While these rules have been helpful one must be conscious of their limitations or risk missing future breakthroughs. Here we describe some of the most commonly stated rules, the reasoning behind their formation, their uses, and limitations.