The study population is the population affected by severe intellectual disability according to the commonly accepted definition (IQ <35), and patients with severe or moderate ID (IQ <50) associated with other clinical signs but no obvious syndromic form known . In 2 cases, the parents of patients will not be affected by DI. The recruitment will be made during genetic counseling. Many patients with DI meet the criteria of the study are already being explored in different centers. For these patients, the DNA of the index case and parents is already available. Families will therefore re-contacted to invite them to participate in this study.