Introduction:

Fragile X syndrome is an inherited disease linked to the X chromosome that mainly affects men and that causes principally intellectual disability; its etiology is a mutation by expansion of CGG trinucleotide repeats in the FMR1 gene located at region Xq27.3.

Case report:

Preschool of six years old with psychomotor delay, characterized by language and cognitive functions corresponding to two years of age; with phenotypic characteristics compatible with SXF, whose karyotype showed presence of mosaicism for 46, and fra (X) (q27.3), with associated problems of upper airways infection secondary to gastroesophageal reflux.

Conclusions:

Fragile X syndrome represents the most common cause of hereditary mental retardation, intellectual disability and autism, being the second most common cause of mental deficiencies genetically associated, after trisomy 21.