PTHLH and breast cancer: The osteolytic factor, parathyroid hormone-related protein (PTHrP), which is expressed in most clinical breast cancer BMET[8,11,59,61–63], was secreted constitutively from ER+ MCF-7 tumor cells used for inoculation, while constitutive PTHrP secretion from ER+ tumor cells isolated from MCF-7 BMET lesions was 2- to 3-fold higher (P ≤ 0.05) [Figure 5B].