Mitochondrial complex III deficiency is one of several conditions caused by dysfunction of mitochondria, which are specialized compartments in cells that generate more than 90% of the energy required by the body. It is a severe, multisystem disorder that includes features such as lactic acidosis, hypotonia, hypoglycemia, failure to thrive, encephalopathy, and delayed psychomotor development. Involvement of internal organs, including liver disease and renal tubulopathy, may also occur. Symptoms typically begin at birth. It is generally caused by genetic changes in nuclear DNA in the BCS1L, UQCRB and UQCRQ genes and inherited in an autosomal recessive manner. However, it may also be caused by genetic changes in mitochondrial DNA in the MTCYB gene, which is passed down maternally or occurs sporadically and may result in a milder form of the condition.