Exclusively females (lethal in males? Or rate of germ cell mutations higher in male germ cells?).  Virtually always sporadic – so not exactly X linked dominant.

MECP2 gene on X chromosome.

Developmental arrest at 6-18 months, then regression, loss of speech, stereotypies esp hands.

Often epilepsy, then complications of severe neurodisability eg chronic lung issues.

Not degenerative however – can live into middle life.