However, mtDNA copy number in the SAT was negatively associated with the expression of the RARRES2 gene in patients without T2DM (with grade III obesity) (r = − 0.84, p < 0.05; r2 = − 0.69, p < 0.05) (Fig. 1x, y). The gene discussed is RARRES2; the disease is type 2 diabetes mellitus.