The present invention relates to a promoter region of a microRNA, the use of a microRNA, in particular miR-21, and related elements for the diagnosis and for the manufacture of a medicament for the treatment and/or prevention of fibrosis and/or fibrosis related diseases. Additionally, the invention concerns antisense oligonucleotides against targets of miR-21. A cell deficient for miR-21, the promoter region and targets of miR-21 and a knock-out organism thereof are also encompassed. Finally, the invention is directed to a method for diagnosing fibrosis and/or fibrosis related diseases and to a method for screening a pharmaceutically active compound for the treatment of fibrosis and/or fibrosis related diseases. The present invention further relates to compositions for use in the treatment, amelioration, and/or prevention of fibrosis. In certain embodiments, the compositions modulate the activity of a miRNA for the treatment, amelioration, and/or prevention of fibrosis. In certain embodiments, the compositions inhibit the activity of miR-21 for the treatment, amelioration, and/or prevention of fibrosis.