Mitochondria, “the powerhouses of the cells” are dynamic organelles which undergo continuous fission and fusion to maintain their diverse cellular functions. Mitochondrial fission factor MFF is a component of the fission machinery shared between mitochondria and peroxisomes. We for the first time present detailed clinical and functional data of three patients with loss of function mutations in MFF and affirm MFF as another disease gene, causing a defect of mitochondrial and peroxisomal homeostasis. Concerning the underlying pathomechanism the consequences of the organellar structural abnormalities still remain unclear and need to be clarified by further studies. (By Dr. Johannes Koch, http://jmg.bmj.com/content/early/2016/01/18/jmedgenet-2015-103500 )