A new article published in the Journal of Toxicologic Pathology reported results of recent studies which suggest an altered expression of microRNAs (miRNAs) in the kidneys during the progression of acute kidney injury (AKI) and chronic kidney disease (CKD) in humans. It was observed that, specifically, miR-21 was strongly associated with renal pathogenesis both in the glomerulus and tubulointerstitium. In addition, adequate evidence indicates the involvement of miRNAs in renal fibrosis. It was stated that owing to the complex morpho-functional organization of the mammalian kidneys, it is essential both to determine the exact localization of the kidney cells that express the miRNAs, and to identify precisely which mRNAs are bound and degraded by these miRNAs. The authors stated that recently, the therapeutic effect of miRNA and/or anti-miRNA administrations have also been demonstrated.