FMR1 and fragile X syndrome: However, they are unstableupon transmission to offspring and, when maternally transmitted, the premutation repeatmay eventually expand to (CGG)>200, thus characterizing the full mutationthat causes the Fragile X syndrome (FXS) as a result of FMRP deficiency.2 FXS is the most common cause ofinherited mental retardation.2,3 FXTAS is not the only conditionassociated with the premutation of the FMR1 gene.