FNDC5 and type 1 diabetes mellitus: 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.