Recently, using an integrated approach of gene expression, congenic/subcongenic strain analysis, quantitative complementation testing and causal analysis, we successfully identified the lymphocyte antigen 75 (Ly75) gene as the causal gene for Pbwg1.5, a QTL that affects resistance to obesity in mice [27]. The gene discussed is LY75; the disease is obesity due to melanocortin 4 receptor deficiency.