The phenotype of trisomy 20p, which results from a parental translocation, includes mild to moderate developmental delay, round face with telecanthus, flat nasal bridge and short palpebral fissures but major pre- and postnatal growth development malformations are rare. The phenotype of interstitial 20p deletion is consistent with Alagille syndrome, an autosomal dominant disease with reduced penetrance and variable expressivity, and is defined by bile duct paucity in association with cardiac, skeletal, ocular and facial abnormalities. 20q13 trisomy shows brachycephaly, hypertelorism, ear anomalies, cleft palate, micrognathia, chin dimple, anteverted nares, cNS malformations, heart defects, psychomotor retardation and reduced life expectancy. Ring chromosome 20 presents with absence of major congenital malformations and paucity of dysmorphic features. Patients can show behavioural problems, seizures, mild dysmorphic features and variable degrees of mental retardation. Sipple syndrome, or multiple endocrine neoplasia type 2, is assigned to chromosome 20p.