McCune-Albright syndrome (MAS) is a rare genetic disorder which is caused by somatic mutations in the GNAS gene. Clinical symptoms of MAS include café-au-lait skin pigmentation, fibrous dysplasia, and autonomous endocrine hyperfunction. Somatic character of the gene defects determines wide variety of syndrome manifestations, from mild forms with minimum presentation to severe conditions with aggressive course. Potential multicomponent form of the MAS syndrome necessitates the dynamic monitoring, including regular screening for possible components of the disease. Therefore, additional methods specifying the diagnosis of MAS syndrome, especially of its suppressed forms, should facilitate selection of patient management strategy and monitoring rate and/or complete exclusion of the diagnosis. Molecular genetic verification of the diagnosis may be one of these methods. Objective — the study was aimed at evaluating massive parallel sequencing (next generation sequencing, NGS) and real-time polymerase chain reaction using the TaqMan technique for detection of somatic mutations (competitive allele-specific TaqMan PCR, CAST-PCR) in the diagnosis of somatic mutations R201C and R201H in the GNAS gene based on DNA obtained from the peripheral blood. Material and methods. The study included patients diagnosed with and suspected for MAS syndrome. Molecular genetic testing of R201C and R201H mutations in the GNAS gene based on DNA extracted from peripheral blood leukocytes was carried out by Next generation sequencing (NGS) and real-time polymerase chain reaction methods using the TaqMan technique for detection of somatic mutations (competitive allele-specific TaqMan PCR, CAST-PCR). Based on clinical data, patients were divided into groups depending on the severity of the disease and the number of MAS manifestations. The results were evaluated by comparing the rate of detected molecular genetic defects in the formed groups of patients. Results. Molecular genetic study included 39 children with MAS syndrome and 6 children with suspected MAS. R201C and R201H mutations in GNAS gene were detected in 16 patients with severe to moderate MAS 16 (41%) 39. No mutations were detected in other MAS patients and patients with suspected MAS. Conclusion. NGS and CAST-PCR methods can detect the presence of mutant alleles R201C and R201H of GNAS gene in DNA samples obtained from the blood in the case of severe to moderate MAS syndrome, but they cannot be recommended for MAS diagnosis based on the peripheral blood samples in children with mild signs of the syndrome or suspected diagnosis.