is a rare inherited degenerative neurological condition that affects the central nervous system. It usually begins in infants between the ages of three months and two years. Rarely, it occurs in teenagers and adults. Signs and symptoms usually progress rapidly. The earliest signs may be poor sucking ability, and the loss of head control and motor skills. These symptoms may be accompanied by loss of appetite, vomiting, irritability, continuous crying, and seizures. As the disorder progresses, symptoms may also include generalized weakness, lack of muscle tone, and episodes of lactic acidosis, which can lead to impairment of respiratory and kidney function. There are several different gene mutations that can cause this condition. Leigh syndrome can be caused by mutations in mitochondrial DNA or nuclear DNA. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.