FGF21 and obesity due to melanocortin 4 receptor deficiency: Davidsohn et al. [22] investigated an in vivo study of three genes—namely, klotho, fibroblast growth factor 21, and a soluble form of the mouse transforming growth factor receptor 2—using an adeno-associated virus and showed that aging-related diseases, including heart failure, obesity, kidney failure, and diabetes, could be improved.