TLR9 and systemic lupus erythematosus: Despite similarities in their downstream signaling cascades, these receptors play opposing roles in most mouse lupus models: TLR7 promotes disease, while TLR9 provides protection — an observation often referred to as “the TLR paradox.” To understand the basis of this dichotomy, Leibler et al. created genetically edited lupus-prone mice in which TLR7 receptors express the TLR intracellular signaling domain (TIR) that corresponds to TLR9, or vice versa.