The main mechanism of pathogenesis fragile X syndrome (FXS) associated with the expansion repeats in the promoter region of the gene, resulting in appearance of fragile site on the locus A of the X-chromosome (FRAXA). The high degree of disabil-ity for mental retardation has both medical and social significance. Medicines for the treatment of patients with FXS do not currently exist. Purpose. The study aimed to determine fragile site FRAXA by molecular cytogenetics in patient's blood samples and to assess the neural activity of different brain regions in patients with FXS and healthy volunteers by resting-state functional magnetic resonance im-aging (fMRI). Materials and methods. Resting-state fMRI was performed on a 1.5 T MRI Achieva scanner (Philips). The study involved patients with confirmed FXS and healthy volunteers. Independent component analysis and seed-based correlation analysis were used to detect default mode network and functional connectivity through a comparison of the patients with healthy volunteers. Results. Chromosome X fragility level in patient's blood samples was determined to be 8% to 19%. The fMRI study showed that a default mode network of the brain in patients with FXS and healthy volunteers does not have statistically significant differences, which may indicate that the basal activity of neurons in patients with FXS is not reduced. In addition, we have found a significant increase in the functional connectivity within the right in-ferior parietal and right angular gyrus in the resting state in patients with FXS. Conclusion. New integrated data of functional status of the brain and genetics in patients with FXS were received. Fragile site FRAXA in the patients' blood samples was deter-mined. The significant increase in the resting state functional connectivity within the right inferior parietal and right angular gyrus in patients with FXS was found.