Therefore, since these ligands and their receptors are molecules involved in the proliferation, survival, migration and possible future BM/bone metastasis of BC cells, studying their simultaneous in vitro expression in human BC cell lines, such as the ER-negative MDA-MB-231 (highly invasive and metastatic cells) and ER-positive MCF-7 (weakly invasive and metastatic cells), offers an approach to a better understanding of their involvement in the evolution of BC. The gene discussed is ESR1; the disease is breast cancer.