The following text lists signs and symptoms of<unk>  and their relative frequency in affected individuals. Consistent (100%) * Developmental delay, functionally severe * Speech impairment, no or minimal use of words; receptive and non-verbal communication skills higher than verbal ones * Movement or balance disorder, usually ataxia of gait and/or tremulous movement of limbs * Behavioral characteristics of the following types: any combination of atypical frequent laughter/smiling; atypically happy demeanor; easily excitable personality, often with hand flapping movements; hypermotoric behavior; short attention span Frequent (more than 80%) * Delayed, disproportionate growth in head circumference, usually resulting in microcephaly (absolute or relative) by age 2 * Seizures, onset usually less than 3 years of age * Abnormal EEG, characteristic pattern with large amplitude slow-spike waves Associated (20–80%)
Angelman syndrome
Cerebral palsy
Gitelman syndrome
Prader–Willi syndrome
FG syndrome
Rett syndrome
Perlman syndrome
Autism spectrum