EXT1 and systemic lupus erythematosus: The primary antigens for a subset of autoimmune diseases, including lupus, are exostosin 1/exostosin 2 (EXT1/EXT2).[4] Exostosins are glycosyltransferases that are involved in the synthesis of complex polysaccharides,[5,6] and they are the primary antigens for MN that is negative for PLA2R and THSD7A.