There are multiple studies proving irisin levels are significantly lower in patients with T2DM (including a meta-analysis published in 2016 and involving 1745 diabetic patients and 1337 controls from 17 cross-sectional and 6 case–control studies [67] and another published in 2021 [68] and evaluating 26 studies with a total of 3667 participants), yet some studies show the opposite correlation [113,130]. The gene discussed is FNDC5; the disease is type 2 diabetes mellitus.