This pro-differentiation function of RANKL on RANK-expressing cells has already been described for several healthy cells, including mammary glands and hair-follicle epithelial cells or pre-osteoclastic cells [32,33], but also in tumors derived from these cell-types, such as breast carcinoma and giant cell tumors in bone [34,35]. The gene discussed is TNFRSF11A; the disease is giant cell tumor.