Using the fixed ratio isobolagram method to determine the interaction index (γ) as described by Tallarida et al. (Tallarida, 1992; Tallarida et al., 1997) the current study demonstrates for the first time that the C. subternata extract, SM6Met, synergistically promotes tamoxifen-induced antagonism of E2-induced ER+ breast cancer proliferation. The gene discussed is ESR1; the disease is breast cancer.