Cytochrome c oxidase deficiency (COX deficiency) is a condition that can affect several parts of the body including the skeletal muscles, heart, brain, and liver. There are four types of COX deficiency differentiated by symptoms and age of onset: benign infantile mitochondrial type, French-Canadian type, infantile mitochondrial myopathy type, and Leigh syndrome. The range and severity of signs and symptoms can vary widely among affected individuals (even within the same subtype and same family) and depend on the form of the condition present. Features in mildly affected individuals may include muscle weakness and hypotonia; in more severely affected individuals, brain dysfunction; heart problems; an enlarged liver; lactic acidosis; and/or a specific group of features known as Leigh syndrome may also be present. COX deficiency is caused by genetic changes in any of at least 14 genes; the inheritance pattern depends on the gene involved.