mIRN21
microRNA 21 also known as hsa-mir-21 or miRNA21 is a mammalian microRNA  that is encoded by the MIR21 gene.
MIRN21 was  one of the first mammalian microRNAs identified. The mature miR-21 sequence is strongly conserved throughout evolution. The human microRNA-21 gene is located on plus strand of chromosome 17q23.2 (5527340955273480) within a coding gene TMEM49 (also called vacuole membrane protein). Despite being located in intronic regions of a coding gene in the direction of transcription, it has its own promoter regions and forms a ~3433-nt long primary transcript of miR-21 (known as pri-miR-21) which is independently transcribed. The stemloop precursor of miR-21(pre-miR-21) resides between nucleotides 2445 and 2516 of pri-miR-21.

# Structure
Gene ID: 406991: "microRNAs (miRNAs) are short (20-24 nt) non-coding RNAs that are involved in post-transcriptional regulation of gene expression in multicellular organisms by affecting both the stability and translation of mRNAs. miRNAs are transcribed by RNA polymerase II as part of capped and polyadenylated primary transcripts (pri-miRNAs) that can be either protein-coding or non-coding. The primary transcript is cleaved by the Drosha ribonuclease III enzyme to produce an approximately 70-nt stem-loop precursor miRNA (pre-miRNA), which is further cleaved by the cytoplasmic Dicer ribonuclease to generate the mature miRNA and antisense miRNA star (miRNA*) products. The mature miRNA is incorporated into a RNA-induced silencing complex (RISC), which recognizes target mRNAs through imperfect base pairing with the miRNA and most commonly results in translational inhibition or destabilization of the target mRNA."

# Mature miR-21
Pri-miR-21 is cut by the endonuclease Drosha in the nucleus to produce pre-miR-21, which is exported into the cytosol. This pre-miR-21 is then cut into a short RNA duplex by Dicer in the cytosol. Although abundance of both strands is equal by transcription, only one strand (miR-21) is selected for processing as mature microRNA based on the thermodynamic stability of each end of the duplex, while the other strand (designated with an asterisk; miR-21*) is generally degraded. Mature microRNA is then loaded into microRNA ribonucleoprotein complex RISC (RNA-induced silencing complex) and guided to target mRNAs with near perfect complimentarily at 3UTR.

# Targets
A number of targets for microRNA-21 have been experimentally validated and most of them are tumor suppressors, Notable targets include:
- ANP32A,
- BTG2,
- Bcl2,
- P12/CDK2AP1,
- HNRPK,
- IL-12p35,
- JAG1,
- MEF2C,
- hMSH2,
- PDCD4,
- PTEN,
- RECK,
- RhoB,
- SMARCA4,
- TGFBRII,
- SPRY1,
- SPRY2,
- TP63, and
- Tropomyosin.

# Functions
"Cystathionine gamma-lyase (CSE) is the major H2S-generating enzyme in vascular smooth muscle cells (SMCs). CSE/H2S system contributes to the maintenance of SMC phenotype, and transcript factor specificity protein-1 (SP1) is a critical regulator of CSE expression during SMC differentiation. The involvements of microRNA-21 (miR-21) in cardiovascular pathophysiology have been known  expression of miR-21 was upregulated in dedifferentiated human aorta SMCs (HASMCs) and injured mouse carotid arteries.  miR-21 expression was upregulated by miR-21 precursor.  miR-21 overexpression significantly repressed the protein expressions of both CSE and SP1, inhibited H2S production, stimulated SMC proliferation, and reduced SMC differentiation marker gene expression, respectively. The mRNA expression of CSE but not SP1 was inhibited by miR-21 precursor. Blockage of SP1 binding by mithramycin or inhibition of CSE activity by DL-propargylglycine did not change miR-21 expression.  miR-21 repressed SP1 protein expression by directly targeting at SP1 3' untranslational regions, which in turn downregulated CSE mRNA expression and stimulated SMC proliferation.  miR-21  in CSE/H2S-mediated-SMC differentiation by targeting SP1."

# Transcriptions
"The miR-21 promoter contains one conserved CArG box21,22 .  MRTF-A regulates miR-21 transcription.  The miR-21 promoter contains one CArG box, which is a binding element for MRTF-A/SRF."

# Vascular smooth muscle cells
Nearly "all  VSMC-restricted contractile protein genes and many other genes important for migration, proliferation, and extracellular matrix production, contain evolutionarily conserved CArG box DNA sequences within their promoters that are required for VSMC transcription in vivo" (Miano 2003).

# Clinical significance

## Cancer
miR-21 is one of the most frequently upregulated miRNAs in solid tumours, and its high levels were first described in B cell lymphomas. Overall, miR-21 is considered to be a typical 'onco-miR', which acts by inhibiting the expression of phosphatases, which limit the activity of signalling pathways such as AKT and MAPK.
As most of the targets of miR-21 are tumor suppressors, miR-21 is associated with a wide variety of cancers including that of breast, ovaries, cervix, colon, lung,  liver, brain, esophagus, prostate, pancreas, and thyroid.  A 2014 meta-analysis of 36 studies evaluated circulating miR-21 as a biomarker of various carinomas, finding it has potential as a tool for early diagnosis.  miR-21 expression was associated with survival in 53 triple negative breast cancer patients. Moreover, it has been demonstrated as an independent prognostic factor in patients with pancreatic neuroendocrine neoplasms.

## Cardiac disease
miR-21 has been shown to play important role in development of heart disease. It is one of the microRNAs whose expression is increased in failing murine and human hearts. Further, inhibition of microRNAs in mice using chemically modified and cholesterol-conjugated miRNA inhibitors (antagomirs) was shown to inhibit interstitial fibrosis and improve cardiac function in a pressure- overload cardiac disease mice model. Surprisingly, miR-21 global knock-out mice did not show any overt phenotype when compared with wild type mice with respect to cardiac stress response. Similarly, short (8-nt) oligonucleotides designed to inhibit miR-21 could not inhibit cardiac hypertrophy or fibrosis. In another study with a mouse model of acute myocardial infarction, miR-21 expression was found to be significantly lower in infarcted areas and overexpression of miR-21 in those mice via adenovirus-mediated gene transfer decreased myocardial infarct size.
miR-21 has been hypothesized to be an intermediary in the effects of air pollution that lead to endothelial dysfunction and eventually to cardiac disease. Expression of miR-21 is negatively associated with exposure to PM10 air pollution and may mediate its effect on small blood vessels.