In summary, the obtained results showed that D-glucuronyl C5-epimerase significantly suppress proliferative activity of breast cancer cells in vitro through the activation of tumour suppressor genes р53, BRCA1, SYK and E2F1 and change of a balance of pro- and apoptotic factors BCL2, NFKB1 and TNF. The gene discussed is BRCA1; the disease is breast cancer.