POLG and External ophthalmoplegia: Dominant mutations in POLG1 instead, are usually associated with adult-onset PEO phenotypes (AD-CPEO) and variable degrees of extra-ocular manifestations such as extrapyramidal signs, spinocerebellar ataxia, peripheral neuropathy, mental retardation, hypogonadism and gastrointestinal dysmotility (‘CPEO plus’) [83,86].