These controversial findings prompted us to analyze, here, diet-induced obesity in a third independent mouse model of mast cell deficiency, in which the absence of mast cells is caused by a principle different from Kit hypomorphic alleles and also from the genotoxic loss of mast cells in Cpa3Cre/+ mice (19, 20). The gene discussed is KIT; the disease is obesity due to melanocortin 4 receptor deficiency.