Schaaf-Yang syndrome (SYS) is a neurodevelopmental disorder caused by pathogenic paternal variants in the MAGEL2 gene, located within the Prader-Willi syndrome (PWS) locus. SYS and PWS present with similar symptoms, including hypotonia, feeding problems, and developmental delay. A distinctive feature is arthrogryposis and SYS patients often have autism. Obesity is present in around 40% of patients, with a later age-of-onset than in PWS.