microRNA-21 (miR-21) is a small non-coding RNA molecule that plays a significant role in regulating gene expression in various cellular processes. Research has shown that miR-21 is upregulated in a variety of cancers, contributing to tumor growth, invasion, and metastasis. It is also implicated in other diseases such as cardiovascular disorders, neurodegenerative diseases, and autoimmune conditions. Studies have focused on understanding the molecular mechanisms of how miR-21 functions in these diseases, as well as exploring its potential as a diagnostic biomarker or therapeutic target. Targeting miR-21 with anti-miR oligonucleotides or other inhibitors has shown promise in preclinical models of cancer and other diseases. Overall, research on miR-21 continues to advance our understanding of its role in various diseases and its potential as a therapeutic target for intervention.