FGF21 and obesity due to melanocortin 4 receptor deficiency: Here, we took advantage of adeno‐associated viral (AAV) vectors and their ability to mediate multi‐year production of therapeutic proteins (Mingozzi & High, 2011; Naldini, 2015) to develop a gene therapy strategy for obesity and T2D based on FGF21 gene transfer to the liver, adipose tissue, or skeletal muscle.