IF1 (ATPase Inhibitor Mitochondrial, Inhibitor of F(1)F(0)-ATPase) is an endogenous inhibitor protein of mitochondrial ATP synthase. The ATP synthase (F1F0-ATPsynthase) is an important enzyme that catalyzes the synthesis of ATP (oxidative phosphorylation) utilizing the energy produced by the trans-membrane electrochemical proton gradient along the respiratory chain. ATP synthase is comprised of the membrane-spanning F0 and the soluble F1 sectors, both of which are multiple protein complexes. IF1 is a basic amphiphilic mitochondrial protein composed of 81 amino acids with a significant degree of homology in various species. This protein is encoded by the nuclear ATPIF1 gene. The gene ATPIF1 lies on the chromosome 1 and it is evolutionarily conserved throughout all eukaryotes, from yeast to mammals. IF1 is mainly located within the mitochondrial matrix. The expression of IF1 in different normal tissues has been shown to vary largely, from very high levels in the heart, to intermediate expression in the liver and negligible levels in breast, colon and lung. IF1 can switch between two different states: an active dimeric form and an inactive tetrameric form, depending on the pH. IF1 forms a dimer at acidic pH (~6.7) and exhibits inhibitory effect, on the other hand, the tetrameric form, which is formed at basic pH (~8.0), cannot interact with ATP synthase. IF1 has the unique capacity to inhibit, through a non-competitive mechanism, the adenosine triphosphate (ATP)-hydrolyzing activity of the F1F0-ATPsynthase without affecting the synthesis of ATP during oxidative phosphorylation. When mitochondria lose proton motive force, the interior of the mitochondria becomes acidic and IF1 reversibly binds to ATP synthase to block wasteful ATP consumption. Under conditions of oxygen deprivation, such as during ischemia or in the presence of an uncoupler of oxidative phosphorylation, the F1F0-ATP synthase can switch from an ATP synthase to an ATPase, hydrolyzing ATP produced in the cytosol by glycolysis. To preserve ATP, C-terminal α-helix of IF1 is inserted into the interface between the α- and β-subunits of F1-ATPase and inhibits ATP hydrolysis. Under aerobic conditions, whether IF1 has a role or what role IF1 plays in the control of the mitochondrial F1F0-ATP synthase/ATPase activity remains poorly understood. IF1 overexpression increased the activity of F1F0-ATP synthase by facilitation of dimerization of F1F0-ATP synthase in aerobic cell culture. IF1 could play an important role in pathology of tissue ischemia and tumor growth by helping to conserve ATP under conditions of oxygen deprivation. Elevated expression is observed in a number of human cancers, including colon, lung, breast and ovarian cancers and hepatocellular carcinoma (HCC). Immunochemical determination of the amount of the natural ATPase inhibitor revealed that the tumor mitochondria contain 2–3 times more ATPase inhibitor than control mitochondria. It is concluded that the low ATPase activity of the tumor mitochondria results from the inhibition of the enzyme activity by the natural ATPase inhibitor. Recent study reported that reciprocal activation between IF1 and nuclear factor NF-κB promoted HCC angiogenesis and metastasis. In colon cancer cells, IF1 promoted aerobic glycolysis and reactive oxygen species-mediated signaling pathway to enhance cell proliferation and cell survival. IF1 may promote tumor progression by promoting migration and invasion in glioma cells.