In addition, FNDC5/irisin functionally interacts with Fibroblast Growth Factor 21 (FGF21), another metabolic regulator, and bioinformatic studies have identified polymorphisms in FNDC5 that could affect this interaction, influencing browning efficiency and susceptibility to metabolic disorders such as obesity and type 2 diabetes [137]. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.