FGF9 and obesity due to melanocortin 4 receptor deficiency: FGF9 is a member of the fibroblast growth factor (FGF) family that mainly binds receptors FGFR2 and FGFR3 [8,9] and plays a crucial role in regulating cell growth, embryonic development, skeletal development [10,11], energy metabolism, and obesity [12,13].