IRS1 and obesity due to melanocortin 4 receptor deficiency: However, as previously observed for PPAR-γ, the effects of IRS-1 may vary depending on ethnicity, presence of obesity, and sample size [77]; therefore, studies conducted on certain Asian populations did not find any association of IRS-1 with insulin resistance and/or risk of T2D [79,80,81] (Table 1).