Angelman syndrome is an example of a disorder involving imprinted genes. Imprinted genes are only expressed from either the maternally or paternally derived member of a homologous chromosome pair. Angelman syndrome is characterized by significant developmental delay or intellectual disability, severe speech impairment, an ataxic gait, and inappropriate happy behavior including excessive laughing and smiling. Other physical concerns include microcephaly, seizures, wide mouth and a prominent mandible. Angelman syndrome is caused by the lack of expression of the maternally inherited region of chromosome 15 (15q11.2-q13). This lack of expression can be caused by a deletion of the maternal chromosome, paternal uniparental disomy (UPD), a mutation in the UBE3A gene in this region, or an imprinting defect.