Angelman syndrome (AS) is a neuro-genetic disorder consisting of severe developmental delay, movement or balance dysfunction, a “happy demeanor” behavioral phenotype (frequent laughter/smiling, hand-flapping, etc.) and minimal or absent speech (with receptive and non-verbal communication skills more pronounced than verbal ones). Frequently (more than 80% of the time), AS is associated with microcephaly, seizures and an abnormal electroencephalogram (large amplitudes, slow spike waves, triphasic waves). Twenty to 80% of AS patients demonstrate clinical features such as tongue thrusting, prognathia, wide- spaced teeth, strabismus, scoliosis and fascination with water.

Clinically, AS in girls during early childhood can mimic the features of the Rett syndrome and in girls with one of these syndromes it may be difficult to differentiate one from another by clinical exam.

Genetically, AS is related to Prader-Willi syndrome as the two syndromes map to the same 15q11.2-13 chromosome region and both conditions are imprinted. However, the conditions are distinct genetically since AS is due to maternal disruption of the maternally-derived UBE3A gene while Prader-Willi syndrome is caused by disruption of multiple genomic elements on the paternally-derived chromosome. So the PWS gene is "switched off" on the maternally inherited chromosome 15. On the other hand, if the deleted area is maternal in origin, the paternal gene is switched off and the patient will have Angelman syndrome (AS).

Each syndrome, when caused by a chromosome deletion of 15q11.2-13 can also result in concomitant deletion of GABRA5, GABRB3 and GABRG3. Thus, production of the GABA receptor may be abnormal. These abnormal GABA receptors have been implicated in AS patient’s unpredictable responses to GABA agonists.