B cell activating factor (BAFF also known as TNFSF13B) and a proliferation inducing ligand (APRIL also known as TNFSF13) are two related members of the tumour necrosis factor ligand super family (TNFSF) that promote B-cell proliferation. BAFF binds to three receptors B-cell activating factor belonging to the TNF family receptor (BAFFR also known as TNFRSF13C ), B-cell maturation antigen (BCMA also known as TNFRSF17) and transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI also known as TNFRSF13B), whereas APRIL interacts with TACI and BCMA. BAFF and APRIL exhibit structural similarity and overlapping yet distinct receptor binding specificity. Both BAFF and APRIL bind BCMA, but the APRIL:BCMA interaction is of higher affinity. TACI binds to both BAFF and APRIL with similar affinity (Day et al. 2005, Marsters et al. 2000, Gross et al. 2000, Rickert et al. 2011). BAFF or APRIL binding to BCMA or TACI promotes B cell development, proliferation, activation, and survival (Moore et al. 1999, Schneider et al. 1999). BAFF and APRIL are secreted as trimers, but BAFF trimers can be ordered into 60-mer arrays (Liu et al. 2003, Lopez-Fraga et al. 2001).