Prader-Willi syndrome is a disorder which affects 1 in 15000 births and is characterised by:

• neonatal hypotonia
• behavioural disturbance, particularly hyperphagia
• mild-to-moderate mental retardation

60% of cases have a small deletion of the paternal chromosome 15q11-13. The majority of the remainder have two apparently normal apparently normal (but maternally inherited) chromosome 15s (See genetics section for more details).