While reduced IRS-1 expression has been reported in muscle of individuals with type 2 diabetes or obese subjects [57], [58], other studies have reported no difference in muscle IRS-1 expression in subjects with polycystic ovary syndrome (PCOS) [59] insulin resistance [60], or type 2 diabetes [61], implying that changes in the insulin signalling pathway downstream of IRS-1 may play a more important role in human insulin resistance. This evidence concerns the gene IRS1 and type 2 diabetes mellitus.