Caused (mostly) by absence of maternal contribution to a region of the 15q chromosome.  Paternal uniparental disomy is one way this can happen, although mostly there is de novo maternal deletion.  The same region is also responsible for Beckwith-Wiedemann syndrome, but this syndrome is the result of a paternal deletion.

Characteristically “happy puppet” –

• severe speech and language delay
• learning disability and epilepsy
• Movement disorder esp ataxia, also tremor, hyperreflexia
• open mouthed expression, large mandible
• Excitability, paroxysmal laughter

[https://www.omim.org/clinicalSynopsis/105830]