We first examined the expression levels of RANK receptor and its ligand, RANKL, in two ERBB2-positive, SKBR3 (ER-negative), BT-474 (ER-positive), and two ERBB2-negative MDA-MB-453 (ER-negative), MCF7 (ER-positive) BC cell lines. The gene discussed is ERBB2; the disease is breast cancer.