The PTH receptor ligand PTHrP, for example, increased bone resorption but was ineffective in increasing the number of mast cells on bone surfaces or inducing peritrabecular bone marrow fibrosis.(29) Osteitis fibrosa is rarely reported in patients with PTHrP-mediated hypercalcemia of malignancy, suggesting that mast cells play a role in mediating the differential skeletal responses to cPTH and PTHrP. This evidence concerns the gene PTHLH and osteitis fibrosa.