Focal bone loss driven by the inflammatory tumor-like synovium in RA and tumor-driven osteolysis in bone-metastatic breast cancer share some common characteristics, including a key role for parathyroid hormone-related protein (PTHrP) secretion from tumor-(like) masses in both processes, which drives RANKL production, and thus osteocyte (Oc) formation and focal bone resorption [49,53]. This evidence concerns the gene PTHLH and rheumatoid arthritis.