RELB(v-rel reticuloendotheliosis viral oncogene homolog B) is also known as IREL. The International Radiation Hybrid Mapping Consortium assigned the RELB gene to chromosome 19. By RT-PCR and immunocytochemical analyses, Clark et al. (1999) showed that RELB expression correlated with dendritic cell activation.NF-kappa-B-inducing kinase is required for osteoclastogenesis in response to pathologic stimuli. Vaira et al. (2008) found that overexpression of Relb, but not Rela, rescued differentiation of mouse Nik -/- osteoclast precursors, indicating that blockade of the alternative NF-kappa-B pathway, rather than the classical NF-kappa-B pathway, is responsible for the osteoclastogenic defect in the absence of Nik. Using Relb -/- mice, they showed that Relb itself was required for Rankl-induced osteoclastogenesis in vitro and for TNF-induced bone resorption in vivo. Both Relb -/- and Nik -/- mice were resistant to tumor-mediated osteolysis. Vaira et al. (2008) concluded that the alternative NF-kappa-B pathway, via RELB, plays an essential and unique role in RANKL signaling toward osteoclast development.