IL1RL1 and obesity due to melanocortin 4 receptor deficiency: Mice with genetic ablation of IL-33 or its receptor ST2 exhibit defective beige adipocyte formation and are more susceptible to develop obesity than their wild-type littermates11,55,56, whilst administration of recombinant IL-33 is sufficient to reduce body weight and adipose mass in both lean and obese mice by increasing energy expenditure11,12.