The small regulatory RNA microRNA-21 (miR-21) plays a crucial role in a plethora 
of biological functions and diseases including development, cancer, 
cardiovascular diseases and inflammation. The gene coding for pri-miR-21 
(primary transcript containing miR-21) is located within the intronic region of 
the TMEM49 gene. Despite pri-miR-21 and TMEM49 are overlapping genes in the same 
direction of transcription, pri-miR-21 is independently transcribed by its own 
promoter regions and terminated with its own poly(A) tail. After transcription, 
primiR- 21 is finally processed into mature miR-21. Expression of miR-21 has 
been found to be deregulated in almost all types of cancers and therefore was 
classified as an oncomiR. During recent years, additional roles of miR-21 in 
cardiovascular and pulmonary diseases, including cardiac and pulmonary fibrosis 
as well as myocardial infarction have been described. MiR-21 additionally 
regulates various immunological and developmental processes. Due to the critical 
functions of its target proteins in various signaling pathways, miR-21 has 
become an attractive target for genetic and pharmacological modulation in 
various disease conditions.