Koolen-de Vries syndrome: a genetic condition caused by changes that eliminate the function of one copy of a genes known as KANSL1. Symptoms include developmental delay and mild to moderate intellectual disability. Other birth defects can be associated with the condition, including a high, broad forehead; drooping of the eyelids (ptosis); narrow eye openings, prominent ears, a bulbous nose, and skin folds covering the inner corner of the eyes (called epicanthal folds). Defects in other organs such as the heart, skeleton, or kidneys, also may be present. It is inherited in an autosomal dominant pattern, meaning that the syndrome is present if one copy of the defective gene is inherited from either parent. Also known as Koolen syndrome.