Complex I (NADH oxidase/Co-enzyme Q reductase) is one of the major sites of electron entry into the mitochondrial electron transport chain (ETC). Complex I catalyzes the 2 electron oxidation of NADH followed by the reduction of ubiquinone (Q) to form ubiquinol (QH2), and ultimately the reduction of the terminal electron acceptor, O2. During the passage of electrons from NADH to Q, the translocation of four protons (H+) from the mitochondrial matrix to the intermembrane space occurs, contributing to the chemiosmotic proton gradient, which is required for oxidative phosphorylation. Cayman's MitoCheck® Complex I Activity Assay allows for the activity of complex I to be determined without the need to isolate mitochondria or pre-incubate with antibodies. The rate of NADH oxidation is measured by a decrease in absorbance at 340 nm and is proportional to the activity of complex I.