What is Angelman Syndrome? Angelman syndrome is a rare genetic disorder that usually manifests in children around two years old. The most common manifestations are mental retardation, severe speech difficulties, malaise, and microcephaly. . The incidence of Angelman syndrome is extremely low, approximately one in 100,000. Although Angelman syndrome is a very rare disease, it is precisely because of its specificity that we can discover the unique beauty of children with this disease. They are often naive