In our analysis, the combined expression of the four genes participating in xenobiotic catabolism (CYP2A6, CYP1A2, CYP3A4, and FMO4) differentiated non-responders from responders with a slightly higher AUC value than any of the separate genes, confirming that CYP enzyme expression in BC is a valuable tool for predicting tumor response to anthracycline–taxane protocols. This evidence concerns the gene CYP3A4 and breast cancer.