Purpose: to identify the role of the polymorphic variant of the CYP2D6*4 gene, as the most common variant among Europeans, in the diagnosis of breast cancer among 240 patients diagnosed with breast cancer and 360 women in the control group of the Chechen population. Quantitative analysis showed that the polymorphic variant CYP2D6 * 4 detection (p=0.005) increased the risk of developing a malignant tumor (OR = 1.65 (95% CI = 1.16 - 2.34). It was shown that in intermediate metabolizers, an ineffective allele breast *4 of the CYP2D6 gene can be a predictor of the development of breast cancer.