Fig. 4. Roles of miRNAs in heart development and function. Diagrams show some of the known roles of muscle-specific miRNAs in heart development and function. Cardiac-specific deletion of Dicer, a ribonuclease III enzyme that is responsible for miRNA maturation, causes dilated cardiomyopathy and a defect in cardiac contractility. miR-1 contributes to numerous heart abnormalities, including arrhythmias, myocyte proliferation and cardiac hyperplasia, ventricular septation defects, and cardiac hypertrophy, whereas miR-133 is associated with arrhythmias and cardiac hypertrophy. In addition, miR-195, mir-208a and miR-21 are implicated in cardiac hypertrophy, and miR-208a can cause a defect in cardiac contractility.