In breast cancer bone metastasis, parathyroid hormone-related peptide (PTHRP) released from tumour cells up-regulates the expression of RANK-L in preosteoblasts, whilst repressing the expression of osteoprotegerin (OPG, which normally acts to inhibit RANK-L function), leading to a stimulation of osteoclasts and consequent bone resorption. The gene discussed is TNFSF11; the disease is neoplasm.