Because IL-13 has been shown to contribute to O3-induced AHR in lean BALB/c mice (Pichavant et al. 2008; Williams et al. 2008), we determined whether there were obesity- and/or TNFR2-dependent differences in IL-13 expression. This evidence concerns the gene TNFRSF1B and obesity due to melanocortin 4 receptor deficiency.