The Coulter counter assay indicated that digitoxin inhibited growth of the MDA-MB-453 and Her2 overexpressing, ER+ BT474 breast cancer cells, with IC50 values of 0.04 μg/ml (0.05 μM) and 0.03 μg/ml (0.04 μM), respectively. The gene discussed is ESR1; the disease is breast cancer.