Ca2+ flux across the mitochondrial inner membrane regulates bioenergetics, cytoplasmic Ca2+ signals and activation of cell death pathways. Mitochondrial Ca2+ uptake occurs at regions of close apposition with intracellular Ca2+ release sites driven by the inner membrane voltage generated by oxidative phosphorylation and mediated by a Ca2+ selective ion channel called the uniporter. Mitochondrial calcium uniporter (MCU) was recently identified as an ion-conducting pore. MCUR1 (mitochondrial calcium uniporter regulator 1), belongs to the CCDC90 family, an integral membrane protein required for MCU-dependent mitochondrial Ca2+ uptake. MCUR1 binds to MCU and regulates ruthenium-red-sensitive MCU-dependent Ca2+ uptake. MCUR1 contains two transmembrane helices, where the N- and the short C-termini are exposed to the inter-membrane space. MCUR1 is ubiquitously expressed in mammalian tissues including: skeletal muscle, brain, heart, lung, liver, kidney, large intenstine and spleen. It was found that cells depleted of MCUR1 have abnormal bioenergetic properties similar to those observed in cells in which Ca2+ uptake is blocked. Thus, MCUR1 is necessary for MCU to efficiently promote mitochondrial Ca2+ uptake and to ensure normal mitochondrial bioenergetics.