Progressive encephalopathy occurring almost exclusively in females. The affected children are generally born clinically normal in most but not all cases. Their neurological and mental development begins to stagnate between the ages of 7 and 18 months with slowing down of the growth of head circumference and a rapid deterioration of brain functions marked by autistic behavior, dementia, apraxia of gait, loss of facial expression, deterioration of purposeful use of hands, ataxia, and severe dementia. There is an intermediate period of relative stable mental status, sometimes lasting several decades, which is followed by further neurological degradation, marked mainly by spasticity of the lower limbs and epilepsy.