CYP3A4 and breast cancer: Moreover, single exposure of 4-hydroxytomoxifen (0.3 and 3 μM) in MCF-7 cells marginally increased the protein expression of CYP3A4 (Figure 1F), which imply that CYP3A4 induction in TAM-resistant breast cancer cells may results from long-term adaption of cells to 4-hydroxytamoxifen.