<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by myoclonic epilepsy with ragged-red fibers ? [/INST] MERRF is a rare condition; its prevalence is unknown. MERRF is part of a group of conditions known as mitochondrial disorders, which affect an estimated 1 in 5,000 people worldwide. </s>