Cytokine that binds to TNFRSF13B/TACI and to TNFRSF17/BCMA. Plays a role in the regulation of tumor cell growth. May be involved in monocyte/macrophage-mediated immunological processes. Homotrimer. The soluble form derives from the membrane form by proteolytic processing. Transgenic mice develop lymphoid tumors that originate from expansion of the peritoneal B-1 B-cell population. Aging transgenic mice develop progressive hyperplasia in mesenteric lymph nodes and Peyer patches, disorganization of affected lymphoid tissues, mucosal and capsular infiltration, and eventual tumor cell infiltration into non-lymphoid tissues such as kidney and liver (PubMed:15488762). Belongs to the tumor necrosis factor family.