ESR1 and breast cancer: This ER+ breast cancer cell line has been used for 50 years and has many sub-clones, such as LM-MCF7, that have demonstrated high metastasis potential, downregulation of p27 (cyclin-dependent kinase inhibitor, also called KIP1, which is a tumor suppressor) expression, and upregulation of bcl-2 (b-cell lymphoma-2 is a regulator protein of cell death) protein expression when injected into severe combined immunodeficiency mice (SCID) [58].