The facial features are considered to be distinctive, characterized by a broad, square face, prominent forehead, broad nasal bridge, and midface hypoplasia. These and other features appear more pronounced with age as in the size of the jaw which is underdeveloped in infancy and eventually becomes prognathic. Most patients have developmental delays, speech and motor deficits, cognitive impairments and behavioral abnormalities. Hypotonia, hyporeflexia, failure to thrive, lethargy, and feeding difficulties are common in infants. Older individuals have REM sleep disturbances with self-destructive behaviors, aggression, inattention, hyperactivity, and impulsivity. Short stature, hypodontia, brachydactyly, hearing loss, laryngeal anomalies, and peripheral neuropathy are common. Seizures are uncommon.