Patients with a good prognosis presented lower miR-21 expression, and the results showed that it was inversely correlated with BRCA1 and BRCA2 expression while directly correlated with Bcl-2 pre and post-chemotherapy, demonstrating that miR-21 could be considered a significant biological marker to evaluate the response of female BC patients to the applied chemotherapy. This evidence concerns the gene BRCA1 and breast cancer.