FMR1 and fragile X syndrome: Soon after the discovery that expansion of the CGG trinucleotide repeat in the fragile X mental retardation 1 (FMR1) gene was the causative mutation of fragile X syndrome (FXS) [1-3], AGG triplets interspersed within the FMR1 allele were observed and hypothesized to stabilize the gene during transmission [4-8] by decreasing the risk of DNA polymerase slippage during DNA replication [9].