ABCB1 and breast cancer: Moreover, tamoxifen, the primary metabolite 4-hydroxy-N-desmethyl tamoxifen, is an ABCB1 substrate [123] widely used in breast cancer therapy (oestrogen receptor-positive subtype); however, 50% of patients develop resistance after 5 years of treatment, increasing risk of metastasis related to an invasive tumour phenotype acquired by epithelial to mesenchymal transition (EMT) [124].