Tumor necrosis factor ligand superfamily member 13B (TNFSF13B) also known as B-cell activating factor (BAFF) is a member of the tumor necrosis factor ligand (TNF) ligand family. Nineteen cytokines have been identified as part of the TNF family on the basis of sequence, functional, and structural similarities. Family members include TNF beta (TNFSF1), TNF alpha (TNFSF2), Lymphotoxin beta (TNFSF3), OX40 Ligand (TNFSF4), CD40 Ligand (TNFSF5), Fas Ligand (TNFSF6), CD27 Ligand (TNFSF7), CD30 Ligand (TNFSF8), 4-1BB Ligand (TNFSF9), TRAIL (TNFSF10), TRANCE/RANKL (TNFSF11), TWEAK (TNFSF12), APRIL(TNFSF13), BAFF (TNFSF13B), LIGHT (TNFSF14), TL1A/VEGI (TNFSF15), and GITR Ligand (TNFSF18). BAFF is expressed as transmembrane protein on various cell types including monocytes, dendritic cells and bone marrow stromal cells. It is expressed in B cell lineage cells, and acts as a potent B cell activator. It has been also shown to play an important role in the proliferation and differentiation of B cells.