Mental retardation (MR), also referred to as learning disability, learning difficulty, intellectual disability, developmental delay, impaired cognition, or mental handicap, is an extremely common condition, affecting about 3% of the population. Virtually all of those individuals with moderate to severe MR [intelligence quotient (IQ) under 50] need lifelong support and about half of those with mild MR (IQ 50-70) are significantly impaired throughout life. Although MR carries with it immense clinical, social, and psychological burdens, the origins remain poorly understood. Its causes tend to be thought of broadly, such as prenatal insult, social disadvantage, and inheritance. As a consequence, it has been impossible to provide effective therapeutic or preventive strategies.

One avenue that has been better pursued in recent years is the exploration of genetic causes. It has long been suspected that small chromosomal rearrangements, unde-tectable by conventional cytogenetic diagnostic methodologies, might be involved in the etiology of MR. However, it is only within the past decade that advances in molecular methodologies and technology have allowed this hypothesis to be tested. Initial studies focused on the ends of chromosomes (the telomeres and adjacent subtelomeric regions) and showed that small, previously undetected rearrangements are a significant cause of unexplained moderate to severe MR, occurring in 7.4% cases. These findings have been corroborated since through a vast number of independent patient studies, and the importance of subtelomeric anomalies in unexplained MR is now well recognized in clinical genetics.

This entry discusses the background to the subtelomeric studies, the methodologies available for testing subtelo-meric regions, the MR subtelomeric studies to date, and how subtelomeric testing has become a widely used tool in clinical diagnostic laboratories, particularly in the diagnosis of unexplained MR. The conclusion addresses the overall impact that subtelomeric testing has had on the diagnosis of MR, the implications for patients and their families, and also future research avenues for exploring the genetic causes of MR and improving our overall understanding of neurocognitive development.

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