TNFSF13B and systemic lupus erythematosus: Belimumab, a monoclonal antibody that binds to the soluble form of the B cell activating factor belonging to the tumor necrosis family (BAFF; also known as B lymphocyte stimulator, BLyS), was approved for the treatment of SLE in 2011 after two successful phase III clinical trials (6, 7); it was the first in history biological agent to be approved for SLE (8), lending credence to the notion that B cells are central in the pathogenesis of SLE (9).