The FMR1 Phenotype: Premutation and Fragile X Syndrome

Fragile X syndrome (FXS) and the fragile X premutation (FXpm) are caused by a mutation on the fragile X messenger ribonucleoprotein 1 (FMR1) gene, which is located on the X chromosome. As these conditions are X-linked, sex differences often present. While there are some similarities between the FXS and FXpm phenotypes, there are many differences. Although the phenotype of FXpm is typically milder than that of FXS, it has become clear that impairment exists both across and within FXS and FXpm. This chapter describes the motor, cognitive, social, and clinical characteristics of individuals with FXS and FXpm across the lifespan as well as possible interventions. Being informed about FMR1 gene mutations is important to support early diagnosis and targeted treatment and intervention for FXS and FXpm. Thus, this chapter highlights the complexity of the FMR1 gene and the importance of understanding how individuals are impacted across the lifespan.