myocyte disarray, absent to severe left ventricular outflow tract obstruction, and distinct septal contours/morphologies with extremely varying clinical course. Mitochondrial complex V deficiency is a shortage (deficiency) or loss of function in complex V of the electron transport chain that can cause a wide variety of signs and symptoms affecting many organs and systems of the body, particularly the nervous system and the heart. The disorder can be life-threatening in infancy or early childhood. Affected individuals may have feeding problems, slow growth, low muscle tone (hypotonia), extreme fatigue (lethargy), and developmental delay. They tend to develop elevated levels of lactic acid in the blood (lactic acidosis), which can cause nausea, vomiting, weakness, and rapid breathing. High levels of ammonia in the blood (hyperammonemia) can also occur in affected individuals, and in some cases result in abnormal brain function (encephalopathy) and damage to other organs. Ataxia, microcephaly, developmental delay and intellectual disability have been observed in patients with a frameshift mutation in MT-ATP6. This causes a C insertion at position 8612 that results in a truncated protein only 36 amino acids long, and two T > C single-nucleotide polymorphisms at positions 8610 and 8614 that result in a homopolymeric cytosine stretch. Hypertrophic cardiomyopathy, a common feature of mitochondrial complex V deficiency, is characterized by thickening (hypertrophy) of the cardiac muscle that can lead to heart failure. The m.8528T>C mutation occurs in the overlapping region of the MT-ATP6 and MT-ATP8 genes and has been described in multiple patients with infantile cardiomyopathy. This mutation changes the initiation codon in MT-ATP6 to threonine as well as a change from tryptophan to arginine at position 55 of MT-ATP8. Individuals with mitochondrial complex V deficiency may also have a characteristic pattern of facial features, including a high forehead, curved eyebrows, outside corners of the eyes that point downward (downslanting palpebral fissures), a prominent bridge of the nose, low-set ears, thin lips, and a small chin (micrognathia). Infantile hypertrophic cardiomyopathy (CMHI) is also caused by mutations affecting distinct genetic loci, including MT-ATP6 and MT-ATP8. An infantile form of hypertrophic cardiomyopathy, a heart disorder characterized by ventricular hypertrophy, which is usually asymmetric and often involves the interventricular septum. The symptoms include dyspnea, syncope, collapse, palpitations, and chest pain. They can be readily provoked by exercise. The disorder has inter- and intrafamilial variability ranging from benign to malignant forms with high risk of cardiac failure and sudden cardiac death.