What is the repeat expansion for fragile X syndrome?

The repeat expansion associated with fragile X syndrome is specifically CGG repeats. In this genetic condition, the FMR1 gene located on the X chromosome exhibits an abnormal number of repeats of the CGG sequence. In a typical population, individuals have between 5 to 44 repeats of this sequence. However, in affected individuals, the number of repeats can expand to over 200. This expansion leads to methylation of the FMR1 gene and subsequent silencing, resulting in the classic symptoms of fragile X syndrome, which include intellectual disability and various behavioral issues.

Understanding the significance of the CGG repeat expansion helps clarify how genetic mutations can lead to specific phenotypic outcomes in diseases. The other repeat types, such as CAG, AGC, and AAA, are associated with different genetic disorders and do not pertain to fragile X syndrome specifically. This highlights the importance of recognizing the unique genetic markers that characterize each condition.