First, we have used a control model to detect an increase in the levels of ER stress markers inducing liver steatosis by a methionine-choline-deficient diet, since it is well established that the UPR is activated in models of obesity and steatosis/steatohepatitis39, and another group of mice with this diet and injected with a viral vector overexpressing GRP78 specifically in the liver33. Here, HSPA5 is linked to obesity due to melanocortin 4 receptor deficiency.