Angelman syndrome (AS) is a genetic condition that causes severe mental retardation, severe speech impairment, and a characteristic happy and excitable demeanor.

Individuals with AS show evidence of delayed development by 6-12 months of age. Eventually, this delay is recognized as severe mental retardation. Unlike some genetic conditions causing severe mental retardation, AS is not associated with developmental regression (loss of previously attained developmental milestones).

Severe speech impairment is a striking feature of AS. Speech is almost always limited to a few words or no words at all. However, receptive language skills (listening and understanding the speech of others) and non-verbal communication are not as severely affected. Individuals with AS have a balance disorder, causing unstable and jerky movements. This typically includes gait ataxia (a slow, unbalanced way of walking) and tremulous movements of the limbs.

AS is also associated with a unique "happy" behavior, which...