UBE3A and Intellectual disability: UBE3A is a maternally expressed gene, so the loss of maternal UBE3A in neurons due to mutation(s) or paternal uniparental disomy of chromosome 15 causes AS, which is characterized by delayed development, intellectual disability, severe speech impairment, ataxia and additional abnormalities (Nicholls and Knepper, 2001; Buiting et al., 2016).