Major clinical features included neurological regression history (100%), somatic growth deficiency (31.2% MECP2 versus 50% CDKL5), microcephaly (100%), loss of spontaneous ambulation (43.7% MECP2 versus 87.5% CDKL5), loss of purposeful hand use (87.5%), scoliosis (37.5% MECP2-mutated patients), absent verbal language (93.7% MECP2 versus 100% CDKL5), absent nonverbal communication (6.2% MECP2 versus 0% CDKL5), respiratory dysfunction (43.7% MECP2 versus 12.5% CDKL5), autonomic nervous system signs (62.5%), stereotypies (100%), and epilepsy (43.7% MECP2 versus 100% CDKL5). Here, MECP2 is linked to epilepsy.