KNOCKOUT NULLS. “Dramatic changes in microRNA levels are typically required to produce a phenotype, and a phenotype is not guaranteed. Even the majority of microRNA knockout models demonstrate a lack of phenotype, except under stressful conditions. This implies that while their loss is generally tolerated under homeostatic conditions, extreme conditions can result in a sufficient breakdown in gene expression control to produce phenotypes.”
http://onlinelibrary.wiley.com/doi/10.1002/stem.1922/full