BCR and Autoimmunity: Deficiency of the Src family kinase Lyn results in widespread autoantibody production and lupus-like disease because Lyn plays a nonredundant negative regulatory role downstream of the BCR by mediating immunoreceptor tyrosine-based inhibitory motif–dependent inhibitory signaling; such autoimmunity is accelerated in Lyn–/–IgD+/– mice due to impaired quiescence of autoreactive B cells regulated by IgD misexpression (17, 19).