Animals and Plants have hundreds of miRNAs with diverse roles in gene regulation. In humans, each miRNA family can control up to several hundred genes (or 500 to be exact, in humans). A loss of function in one, can lead to array of developmental defects. Similarly in plants, an miRNA mutant can have a variety of phenotypes. However, interestingly, many miRNAs only have one target, which is frequently a transcription factor that in turn, controls many genes itself. It’s really like a house of cards.