This syndrome is characterised by psychomotor delay and severe myopathy (hypotonia, absent tendon reflexes and delayed myelination) from birth, associated with hypermethioninaemia and elevated serum creatine kinase levels. It has been described in three unrelated patients. Transmission appears to be autosomal recessive. Two causative mutations have been identified in the gene encoding S-adenosylhomocysteine hydrolase (SAHH; AHCY), an enzyme involved in methionine metabolism. A methionine-restricted diet, together with creatine supplements, may partly improve the delayed myelination and psychomotor development.