TACI, Transmembrane Activator CAML (calcium modulator and cyclophilin ligand) Interactor, is a 32 kD type III transmembrane protein. It is a member of the TNF receptor superfamily known as TNFRSF member 13B (TNFRSF13B) or CD267. TACI is expressed on B cells and myeloma cells. TACI contains 2 cysteine-rich domains (CRDs). Several proteins (BAFF/BLys, APRIL, Syndecan-2) have been identified as TACI ligands. The interaction of TACI with its ligands induces the activation of the transcription factors NFAT, AP1 and NF-κ B and plays a crucial role in humoral immunity through negative regulation of B cell proliferation and survival.