MiR-138
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. Species distribution. The presence of miR-138 has been detected experimentally in humans ("Homo sapiens") and in different  animals including house mouse ("Mus musculus"), brown rat ("Rattus norvegicus"), platypus ("Ornithorhynchus anatinus"), Carolina anole("Anolis carolinensis"), cattle ("Bos taurus"), common carp ("Cyprinus carpio"), dog ("Canis familiaris"), Chinese hamster ("Cricetulus griseus"), zebrafish ("Danio rerio"), red junglefowl ("Gallus gallus"), western gorilla ("Gorilla gorilla"), gray short-tailed opossum ("Monodelphis domestica"), "Oryzias latipes", sea lamprey ("Petromyzon marinus"), Tasmanian devil ("Sarcophilus harrisii"), wild boar ("Sus scrofa") and zebra finch ("Taeniopygia guttata"). It is also predicted computationally that the miR-138 gene exists in the genome of other animals including horse ("Equus caballus"), rhesus macaque ("Macaca mulatta"), takifugu rubripes ("Fugu rubripes"), Bornean orangutan ("Pongo pygmaeus"), chimpanzee ("Pan  troglodytes"), "Tetraodon nigroviridis" and western clawed frog ("Xenopus tropicalis"). Genomic location. In human genome, there are two miR-138 associated genes and they are not located in any cluster. More precisely, the miR-138-1 gene is in region 5 at 3p21.3 and miR-138-2 is located on chromosome 16 (16q13). Pattern of expression. In adult mice, miR-138 is only expressed in brain tissue. Its expression is not uniform throughout the brain but restricted to distinct neuronal populations. On the contrary, its precursor, pre-miR-138-2, is ubiquitously expressed throughout all tissues, which suggests that the expression of miRNAs can be regulated  at the post-transcription level. In the zebrafish, miR-138 is expressed in specific domains in the heart and is required to establish appropriate chamber-specific gene expression patterns. Targets and function. Since the identification of miR-138, a number of targets have been found and some of them have been verified experimentally. It has been proven that miR-138 is involved in different pathways. Furthermore, it is in relation with various types of cancer.