Small, ~22-nt, RNA molecule that is the endogenous transcript of a miRNA gene. Micro RNAs are produced from precursor molecules (SO:0000647) that can form local hairpin structures, which ordinarily are processed (via the Dicer pathway) such that a single miRNA molecule accumulates from one arm of a hairpin precursor molecule. Micro RNAs may trigger the cleavage of their target molecules or act as translational repressors.