Prader-Willi syndrome (PWS) is characterized by severe hypotonia, poor appetite, and feeding difficulties in early infancy, followed in early childhood by excessive eating and gradual development of morbid obesity (unless food intake is strictly controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone). A distinctive behavioral phenotype (temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Characteristic facial features, strabismus, and scoliosis are often present.

Hypomagnesemia, seizures, and impaired intellectual development-1 (HOMGSMR1) is characterized by onset of seizures associated with low serum magnesium in the first year of life. Affected individuals show variable degrees of delayed psychomotor development (summary by Arjona et al., 2014). Genetic Heterogeneity of Hypomagnesemia, Seizures, and Impaired Intellectual Development HOMGSMR2 (618314) is caused by mutation in the ATP1A1 gene (182310) on chromosome 1p13.