The highlights include (i) the consistent downregulation of circulating irisin in both T1DM and T2DM; (ii) the positive relationship between circulating irisin and subcutaneous fat FNDC5 expression, but not epididymal fat or muscle in T1DM; (iii) the negative correlation between subcutaneous fat FNDC5 expression and metabolic parameters, such as blood glucose and lipid levels, in both T1DM and T2DM. The gene discussed is FNDC5; the disease is type 2 diabetes mellitus.