Inactivation of cathepsin B (ctsb) protects against diet-induced fatty liver disease, as ctsb−/− mice placed on a high carbohydrate diet developed marked obesity, but not the severe hepatomegaly, steatosis and associated dysmetabolic syndrome observed in the wild type (WT) controls placed on the same diet [14]. Here, CTSB is linked to obesity due to melanocortin 4 receptor deficiency.