FMR1 and fragile X syndrome: This results in a deficit of the FMR1-encoded protein fragile X messenger ribonucleoprotein 1 (FMRP), an RNA-binding protein that regulates dendritic translation and plays a critical role in synaptic development and function [2] FXS can also be caused by mosaicism of transcriptional silencing of the gene, occurring in some but not all of the cells due to either varying size of the repeat expansion or variations in methylation patterns.