Prader-Willi syndrome (PWS) is characterised by hypotonia (floppiness), feeding problems and failure to grow as a baby. This is followed by excessive or rapid weight gain due to an insatiable appetite, which usually starts at about 2-3 years of age. The affected person also has characteristic facial features and intellectual disability may occur. Behavioural problems are common and can include temper tantrums, obsessive/compulsive behaviour, and skin picking. All individuals with Prader-Willi syndrome have some cognitive impairment, which ranges from borderline normal, to mild intellectual impairment. PWS occurs when the genes in a specific region of chromosome 15 do not function. This region of chromosome 15 is located at 15q11.2-q13 and has been designated the Prader-Willi syndrome/Angelman syndrome (PWS/AS) region. PWS is usually sporadic (a spontaneous or new genetic change) but sometimes it may be inherited.