Specifically, it seems obvious that circulating irisin levels are lower in subjects or animal models with diabetes mellitus because a number of studies indicated that serum irisin was reduced in T2D [11,23,24,25,26,27,28], high fat diet- induced T2D mice [9], and streptozotocin-induced type 1 diabetic mice models [29]. The gene discussed is FNDC5; the disease is type 2 diabetes mellitus.