A subset of gp120-binding antibodies was shown to hydrolyze gp120 by a mechanism analogous to serine protease [133], As the nucleophilic region responsible for this activity was localized in the light chain [134,135], a library of light chains prepared from three lupus patients [132] was screened with an electrophilic analogue of gp120 residues 421–433 to isolate antibodies capable of binding and hydrolyzing gp120 [75]. This evidence concerns the gene ITIH4 and systemic lupus erythematosus.