Taking this in to consideration, Davidsohn and colleagues undertook an in vivo study to evaluate gene therapies based on three longevity associated genes (fibroblast growth factor 21 [FGF21], Klotho, the soluble form of the mouse transforming growth factor receptor 2 [sTGFβR2]) delivered individually and in combination using AAV, and explored their ability to alleviate four age-related diseases—obesity, type II diabetes, heart failure, and renal failure [141]. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.