Mental retardation is a common and distressing disorder that affects 3% of the population. It is a clinically and etiologically heterogeneous group of conditions. It is among one of the major disorders that are still poorly understood in terms of etiopathogenesis and for which very little therapeutic help is provided. Poor understanding of its origins hampers the provision of effective treatment and preventive regimens therefore mental retardation remains a major challenge for medical practice.It could be due to combination of genetic and environmental factors. As Fragile X is the biggest reason behind X linked mental retardation hence it might be a significant contributor to idiopathic mental retardation. Because of unique structural and functional characteristics of telomere associated sites such as in XLMR (FRX), its difficult to find the locus of mutation by conventional cytogenetics. Hence molecular techniques may be useful. Determining a specific etiologic diagnosis is central to understand the nature of the problem. It is very helpful in providing answers to questions regarding prognosis, recurrence risk and directing specific therapy.