Intellectual Disabilities

Background: \- A number of rare inherited diseases affect only a few patients, and the genetic causes of these conditions remain unknown. Researchers are studying the use of a new technology called genome sequencing to learn which gene or genes cause these conditions. Understanding the genes that cause these diseases is important to improve diagnosis and treatment of affected patients. Objectives: * To identify the genetic cause of disorders that are difficult to identify with existing techniques. * To develop best practices for the medical and counseling challenges of genome sequencing. Eligibility: * Individuals who have one of the rare disorders under consideration in this study. These conditions are generally those in which the genetic cause of the disorder is unknown. The eligibility of most individual participants will be decided on a case-by-case basis by the researchers. * Family members of affected individuals, if that family member (often a parent) may provide genetic information. Design: Participants in this study will have at least one and in some cases several of the following procedures: * A medical genetics evaluation. * Other tests that may include x-rays, m