To assess the activity of G1T48 on endocrine refractory ER mutants, we utilized a reporter gene assay in ER-negative SKBR3 breast cancer cells transfected with ER expression vectors (wtER or the two most common ER mutants, ER-Y537S or ER-D538G) (Fig. 4a). This evidence concerns the gene ESR1 and breast carcinoma.