LEP and obesity due to melanocortin 4 receptor deficiency: Similarly to the ob murine model, db/db mice (i.e. mice that lack functional Ob-Rb isoform due to a mutation in the cytoplasmic domain of the receptor) and carboxypeptidase E-deficient (CPEfat) mice (i.e. a strain characterized by obesity, resulting from a functional mutation in the gene encoding carboxypeptidase, and increased leptin levels) present increased baseline airway responsiveness, as well as augmented responses to O3 exposure, when compared to their lean controls [119,120].