To determine the effect of enhanced PTK6 on growth of ER+ breast cancer cells, we overexpressed active PTK6 (MF-PTK6) in ER+ MCF-7 and T47D breast cancer cell lines, which are relatively sensitive to tamoxifen and undergo growth arrest in response to tamoxifen treatment.30 Expression of MF-PTK6 in MCF-7 and T47D results in active PTK6, as indicated by enhanced levels of autophosphorylated PTK631 (Fig. 1b, c). Here, ESR1 is linked to breast cancer.