Cytochrome oxidase (Cytochrome Oxidase, COX) is considered an enzyme inherent to the mitochondrial membrane and is highly active in cells containing a large number of mitochondria (such as cardiac muscle, renal tubular epithelium, gastric wall cells, and hepatocytes). This enzyme activity often serves as an indicator of intracellular oxidative metabolism and as one of the marker enzymes of mitochondria. Succinate dehydrogenase (succinate dehydrogenase, SDH) is the first enzyme of the succinate oxidase line, located in the mitochondria. Succinate dehydrogenase is a flavoprotease, containing-SH in the molecule, which determines the activity of the enzyme, so all closed-SH can be used as inhibitors. This enzyme participates in the tricarboxylic acid cycle. In histochemistry, the enzyme activity is often used as the representative of the tricarboxylic cycle and also as one of the marker enzymes of mitochondria. Cytochrome oxidase and succinate dehydrogenase are sensitive to fixative, so quick frozen fresh tissue is needed.

COX-SDH Stain Kit can be used to show COX negative fibers, COX negative position may be SDH positive, 3 of 13 subunits of COX are encoded by mitochondrial DNA, while SDH is encoded by nuclear DNA, so SDH is not affected by mitochondrial DNA mutations, and mitochondrial myopathy fragmentation red fibers are generally COX negative. The combined COX-SDH staining with this kit allows for efficient screening for mitochondrial disease and for myofiber typing.