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 (55273409–55273480) 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 transcription of miR-21 (known as pri-miR-21) which is independently transcribed. The stem–loop recursor of miR-21(pre-miR-21) resides between nucleotides 2445 and 2516 of pri-miR-21.
MiR-21 is one of the first microRNA to be described as an oncomir. 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.