Loss of maternally inherited allele from chromosome 15.Gene from dad is normally silent (paternal imprinting) and maternal gene is deleted/mutated.

Etiology:- 5% of cases due to paternal uniparental disomy- Maternal microdeletion

- Affects males and females

Clinical features:- Poor motor and language development- Inappropriate laughter/smiling ("happy puppet")- Epilepsy/seizures- Ataxic gait/tremors- Intellectual disability