A chromosomal anomaly characterized by developmental delay, childhood hypotonia, facial dysmorphism, and a friendly/amiable behavior. The disease is MONDO_0012496 (Koolen-de Vries syndrome). Also known as: KANSL1-related intellectual disability syndrome, KDVS, KdVS, Koolen de Vries syndrome, Koolen-De Vries syndrome, chromosome 17q21.31 deletion syndrome, microdeletion 17q21.31 syndrome.