Fragile X syndrome

Robin Godshalk MS MHA LGC CGC (

Dr. Godshalk of Fragile X Center at Atlantic Health System in Morristown, New Jersey has no relevant financial relationships to disclose.

Bernard L Maria MD, editor. (

Dr. Maria of Thomas Jefferson University has no relevant financial relationships to disclose.

Originally released March 25, 1994; last updated January 6, 2021; expires January 6, 2024


Fragile X syndrome is a classic neurologic disease with unique manifestations on the clinical and molecular level. It is the most common genetic cause of cognitive impairment in males and can render symptoms of speech and developmental delay. Further, it represents one of the many important inherited diseases that are caused by expansions or amplification of short DNA repeats on specific genes. This article will help elucidate the significance of the fragile X syndrome as well as fragile X tremor-ataxia syndrome (FXTAS) for both younger and older patients and will provide insight on the potential benefits of a diagnostic workup and particular situations where a screening may be beneficial. It will also point to current research that has led to possible future treatments for symptoms of fragile X syndrome.

Historical note and terminology

Fragile X syndrome is the most common inherited cause of cognitive impairment in males. It is named for the folate sensitive fragile site Xq27.3 and is caused by an expansion or amplification of a CGG trinucleotide repeat in the first exon of the fragile X mental retardation gene (FMR1), located on the long arm of the X chromosome. It affects males more severely and more frequently than females. It is a paradigm of a neurologic disease with a well-recognized phenotype and a clearly defined genetic and molecular basis that explains a complex mode of inheritance and the associated phenomenon of anticipation.

Martin and Bell first described this syndrome in 1943 (Martin and Bell 1943). In 1969, Lubs was the first to demonstrate a fragile site on the X chromosome (Lubs 1969). In 1977, Sutherland showed this to be a reliable finding in cells cultured in a folate deficient medium (Sutherland 1977). In 1980, Turner and colleagues recognized the combination of macroorchidism and cognitive impairment in males in conjunction with a fragile site to be a distinct clinical entity (Turner et al 1980). Verkerk and colleagues discovered the molecular basis of the condition in 1991 (Verkerk et al 1991).

In 2001, Hagerman and colleagues first recognized Fragile X-associated tremor/ataxia syndrome (FXTAS), a late-onset progressive neurologic disorder. FXTAS develops in a subset of fragile X premutation carriers and involves gait ataxia, action tremor, parkinsonism, peripheral neuropathy, autonomic disorders, cognitive impairment (Hagerman et al 2001), and essential tremor (Gorman et al 2008). The elevated mRNA in fragile X premutation carriers are vulnerable to neurotoxin, leading to early cell death and brain disease, consistent with FXTAS exhibiting neuropsychiatric and neurologic symptoms (Paul et al 2010).

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