The role of mir-21 in the development of cancer is so well established that it was studied in different diseases such as lung cancer: in this case, the upregulation of miR-21 represented a pro-tumorigenic factor, inducing the RAF/MEK/ERK pathway, dedicated to the regulation of the cell cycle, and increasing the production of IL-8 [33]. The gene discussed is MAP2K7; the disease is lung cancer.