Therefore, carefully designed prospective studies are needed, jointly assessing PK and clinical endpoints (breast cancer progression/recurrence, adverse drug reactions) and/or an appropriate biomarker [e.g., tumor size, circulating tumor cells, Ki67 antigen (Yerushalmi et al., 2010) over time (Ribba et al., 2014; Bender et al., 2015; Buil-Bruna et al., 2016)] in a representative, well-defined tamoxifen patient population. Here, MKI67 is linked to breast cancer.