20q13.33 is a rare chromosomal anomaly syndrome resulting from the partial deletion of the long arm of chromosome 20 with a highly variable phenotype typically characterized by hypotonia, intellectual disability, cognitive and language deficits (including decreased or absent speech), pre and post-natal growth retardation, feeding difficulties, microcephaly, and malformed hands and feet. Neurodevelopmental disorders (including hyperactivity, social interactive problems and autism spectrum disorder), seizures and dysmorphic facial features (high forehead, hypertelorism, malformed ears, broad nasal bridge, bulbous nasal tip, thin upper lip, small chin) are frequently associated. The disease is Monarch Disease Ontology identifier MONDO_0016843 (20q13.33 microdeletion syndrome). Also known as: Del(20)(q13.33), monosomy 20q13.33.