ESR1 and breast cancer: In addition to its common expression in clinical BMET, PTHrP has been a molecule of interest in the pathogenesis of osteolytic breast cancer BMET since being identified as a primary driver of metastatic progression in the first preclinical breast cancer BMET model described over two decades ago, which utilized bone tropic ER- MDA-MB-231 cells, variants of which have remained a mainstay of preclinical BMET research to the present day [14,15,19,20].