<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by Koolen-de Vries syndrome ? [/INST] The prevalence of Koolen-de Vries syndrome is estimated to be 1 in 16,000. However, the underlying genetic cause is often not identified in people with intellectual disability, so this condition is likely underdiagnosed. </s>