Fragile X-related disorders (FXDs), also known as FMR1 disorders, are examples 
of repeat expansion diseases (REDs), clinical conditions that arise from an 
increase in the number of repeats in a disease-specific microsatellite. In the 
case of FXDs, the repeat unit is CGG/CCG and the repeat tract is located in the 
5' UTR of the X-linked FMR1 gene. Expansion can result in neurodegeneration, 
ovarian dysfunction, or intellectual disability depending on the number of 
repeats in the expanded allele. A growing body of evidence suggests that the 
mutational mechanisms responsible for many REDs share several common features. 
It is also increasingly apparent that in some of these diseases the pathologic 
consequences of expansion may arise in similar ways. It has long been known that 
many of the disease-associated repeats form unusual DNA and RNA structures. This 
review will focus on what is known about these structures, the proteins with 
which they interact, and how they may be related to the causative mutation and 
disease pathology in the FMR1 disorders.