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 lymphoma,[20] breast,[21] ovaries,[22] cervix,[23] colon,[12] lung,[24] liver,[13] brain,[25] esophagus,[26] prostate,[24] pancreas,[24] and thyroid.

[27] In 2010, it was develop the first-in class in vivo model where a non-coding RNA (including a microRNA) is able to create and maintain a tumor[20] in the first described onco-miRNA adicction.

[28] A 2014 meta-analysis of 36 studies evaluated circulating miR-21 as a biomarker of various carcinomas, finding it has potential as a tool for early diagnosis.

[35] 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.