SLITRK4 and fragile X syndrome: For increasing the understanding of FXS etiology, the expression of four genes adjacent to the CGG repeats expansion of the FMR1 gene (i.e., SLIT and NTRK-like protein 2 (SLITRK2), SLIT and NTRK-like protein 4 (SLITRK4), gamma-aminobutyric acid receptor subunit alpha-3 (GABRA3), and methyl CpG binding protein 2 (MECP2)) were evaluated in the females with FMR1 gene full mutation and different phenotypic characteristics.