Evidence from observational case–control studies has demonstrated a positive association between circulating IL-6 and RA.[3–7] However, it is not clear whether this association is causal, because of the involvement of many confounding factors, such as BMI, age, and genetic backgrounds, as well as the complex biological effects of IL-6, which are in tractable in classic epidemiology. Here, IL6 is linked to rheumatoid arthritis.