ESR1 and breast cancer: Using the combinatorial MTT assay, which is similar to the checkerboard assay, the most commonly used method in antibiotic, anti-microbial, immune disease and viral infection studies to validate improved effectiveness of a drug combination (veldstra, 1956; White et al., 1996; Doern, 2014), the current study showed that the degree of inhibiting E2-induced ER+ breast cancer cell proliferation was increased by combining 4-OH-Tam with SM6Met.