Cellular growth inhibition studies show that SP-1-303 differentially inhibits growth of estrogen receptor positive breast cancer (ER+ BC) cells with effective growth inhibition concentrations (EC50) for MCF-7 and T47D cells ranging from 0.32 to 0.34 μM, compared to 1.2–2.5 μM for triple negative breast cancer cells, and ~12 μM for normal breast epithelial cells. This evidence concerns the gene ESR1 and breast cancer.