MicroRNAs are molecules which can prevent messenger RNAs from being translated into proteins by ribosomes. When microRNAs bind to a messenger RNA molecule, they can either prevent it from being translated into protein or cause it to be degraded, depending on the specific microRNA and mRNA involved. Following resistance training, certain microRNAs are upregulated or downregulated (meaning their production is increased or decreased). The presence or absence of certain microRNAs can adjust the rate of muscle protein synthesis up or down. But it turns out that the expression of microRNA varies between individuals. High responders adjust the production of certain microRNA molecules in a pattern that is more favorable for muscle development, maintenance, and repair. For example, a 2011 study by Davidsen and colleagues found that high responders maintained high levels of miR-378, miR-29a, and miR-26a following training whereas low responders did not. Those microRNAs promote muscle protein synthesis, reduce protein breakdown, and increase ribosome biogenesis.