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Introduction. Breast cancer is the leader in cancer incidence in theRussian Federation. The tumor is considered extremely heterogeneous and the luminal subtypes of breast tumors occupy a special place, since they are considered relatively favorable in therapy and control of the disease.Drug therapy for hormone-positive cancer has undergone significant evolution and new anticancer agents have appeared in the arsenal of the oncologist and have shown promising results compared to classical therapy. The search for predictive markers of the effectiveness of new therapy has become of great importance. This marker turned out to be a mutation in the PIK3CA gene – one of the most frequent genetic disorders in breast cancer cells. According to the literature, the presence of this mutation negatively effects on endocrine therapy for breast tumors.The aim of this study was to analyze the frequency of mutations in the PIK3CA gene among patients with hormone-positive tumors, and the effectiveness of therapy with CDK4/6 inhibitors in this group of patients.Materials and methods. The material for the study of the mutation in the PIK3CA gene was tumor biopsies of 31 patients and clinical data on the response to therapy with CDK4/6 inhibitors and classical hormone therapy.Results and discussion. The results of the work showed a high incidence of the PIK3CA mutation among hormone-positive tumors (45%). The mutation resulted in a decrease in both the median time to progression after radical surgery (from 48.4 ± 7.8 months to 30.1 ± 6.0 months) in patients receiving adjuvant hormone therapy and progression-free survival in patients receiving therapy with CDK4 /6 inhibitors (4.2 months versus 9 months). This confirmed the theory that the PIK3CA mutation negatively affects the outcome of hormone therapy.Conclusions. PIK3CA is an important predictive marker in endocrine therapy for hormone-positive tumors. Its presence not only determines the relatively worse results of treatment, but can also serve as an indication for the appointment of a special series of drugs – inhibitors of this mutation.