The pivotal regulator of osteoclastic activity is the system osteoprotegerin (OPG)/receptor activator of NF-κB (RANK)/RANK-ligand (RANKL) (Hofbauer et al, 2004), which is involved in physiological as well as pathological conditions, such as metastatic bone disease and multiple myeloma (Chikatsu et al, 2000; Lipton et al, 2002; Park et al, 2003; Vanderkerken et al, 2003). This evidence concerns the gene TNFRSF11A and plasma cell myeloma.