miR-138 is a family of microRNA precursors found in animals, including humans. MicroRNAs are typically transcribed as ~70 nucleotide precursors and subsequently processed by the Dicer enzyme to give a ~22 nucleotide product. The excised region or, mature product, of the miR-138 precursor is the microRNA mir-138. miR-138 has been used as an example of the post-transcriptional regulation of miRNA, due to the finding that while the precursor is expressed ubiquitously, the mature product is found only in specific cell types.