Mice overexpressing the propeptide show both increased muscle fiber size and number (28) and resistance to diet-induced obesity (60), whereas we have previously shown that intravenous administration of adeno-associated virus (AAV)-8-myostatin propeptide (ProMyoAAV) leads to a generalized dose-dependent increase in muscle mass (12, 32). Here, MSTN is linked to obesity due to melanocortin 4 receptor deficiency.