<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by megalencephalic leukoencephalopathy with subcortical cysts ? [/INST] Megalencephalic leukoencephalopathy with subcortical cysts is a rare condition; its exact prevalence is unknown. More than 150 cases have been reported in the scientific literature. </s>