To explore the potential anti-cancer effects of the secretome from MDEC from domesticated mammals with a low cancer incidence, the normal human breast epithelial cell line MCF10a, the estrogen receptor-positive (ER+) breast cancer cell line MCF-7, and the triple-negative breast cancer (TNBC) cell line MDA-MB-231 were incubated with conditioned medium (CM) from equine MDEC (eqMDEC) and changes in cell metabolism, as a proxy for cell viability, were assessed using MTT assays. The gene discussed is ESR1; the disease is breast cancer.