The clinical features of Angelman syndrome (AS) include microcephaly, severe mental retardation, "puppet-like" ataxic gait with jerky arm movements, hyperactivity, bouts of inappropriate laughter, EEG abnormalities, and seizures. The frequency of occurrence of AS is in the range of 1/10,000 to 1/20,000 births. The AS locus maps to the imprinted chromosome 15q11-q13 region and the disease is caused by the absence of a normal maternal contribution to this region. The genetic complexity of AS is revealed by the existence of at least four molecular classes. A candidate AS gene, ubiquitin protein ligase 3A (UBE3A/E6-AP), has been identified, and mutations of this gene have been detected in several cases of AS. Moreover, UBE3A is expressed predominantly from the maternal allele in brain, strongly supporting its causative role in AS. However, there is evidence to suggest that, in addition to UBE3A, another gene(s) may be involved either directly in AS and/or indirectly by regulating UBE3A expression.