Mitochondrial inner membrane calcium uniporter that mediates calcium uptake into mitochondria. Constitutes a pore-forming and calcium-conducting subunit. Mitochondrial calcium homeostasis plays key roles in cellular physiology and regulates cell bioenergetics, cytoplasmic calcium signals and activation of cell death pathways. A number of isoforms are produced. According to EST sequences. Belongs to the MCU (TC 1.A.77) family. Truncated N-terminus.