Koolen de Vries syndrome is a disorder characterized by developmental delay, mild to moderate intellectual disability, congenital malformations, and behavioral features. Other problems include weak muscle tone (hypotonia) in childhood, recurrent seizures (epilepsy), and distinctive facial features. Males with Koolen de Vries syndrome often have undescended testes (cryptorchidism). Other symptoms may include defects in the walls between the chambers of the heart (septal defects) or other heart defects, kidney problems, and skeletal anomalies such as foot deformities. It is caused by genetic changes in the KANSL1 gene, or by the loss of a small amount of genetic material in chromosome 17 that includes the KANSL1 gene (chromosome 17 q21.31 microdeletion). Inheritance is autosomal dominant.