Microscopically, GCTB are typically comprised of RANK-positive circular mononuclear cells, “reactive” rich RANK-positive multinucleated giant cells, “neoplastic” densely cellular spindled RANKL-positive stromal-like tumor cells, areas of sparse osteoid matrix and woven bone (16, 18). The gene discussed is TNFRSF11A; the disease is neoplasm.