Molecular analysis of residual tumor after administration of neoadjuvant chemotherapy revealed several mechanisms of resistance to treatment. High frequency of RAS-RAF-MEK-ERK signal pathway activation was found in cells of triple negative breast cancer after neoadjuvant chemotherapy, which correlated with high proliferation index and low pathological complete response rate. Thus, inhibition of MEK molecule - an intermediary transducer of this pathway may decrease activity of this pathway and restore tumor sensitivity to cytostatic agents.