MyD88 signalling is critical for defence against some microbial pathogens, but is a “double-edged sword” because the overproduction of inflammatory mediators, and/or failure to terminate inflammation once it has done its job, is a cause of inflammatory and autoimmune diseases, such as arthritis, asthma, colitis, fibrosis, lupus, psoriasis and sepsis. This evidence concerns the gene MYD88 and systemic lupus erythematosus.