mitochondrial complex V deficiency, nuclear type 6 (EFO_0010656) is an autosomal recessive mitochondrial disorder characterized by gross motor developmental delay manifesting in the first years of life, and subsequent episodic developmental regression. The episodes are associated with metabolic stress, including fever, illness, and general anesthesia. Patients develop gait difficulties or loss of ambulation, as well as other variable abnormalities, including abnormal movements, hemiplegia, and persistent lethargy. Brain imaging shows degenerative features in the basal ganglia and brainstem consistent with a diagnosis of Leigh syndrome. Also known as: mitochondrial complex V (ATP synthase) deficiency, nuclear type 6.