GRK2 and obesity due to melanocortin 4 receptor deficiency: We have previously reported that GRK2 levels increase in several key metabolic tissues in experimental conditions promoting insulin resistance (IR) and that GRK2+/− mice (the use of GRK2−/− animals is not experimentally possible since they die during embryogenesis [33]) are protected against high-fat diet (HFD)-induced obesity and IR.