IRS1 and obesity due to melanocortin 4 receptor deficiency: IRS1 mSer612 and mSer632/635 have been cited as negative regulatory sites for IRS1 signaling through tyrosine phosphorylation [5], and mSer1097, regarded as a potential mammalian target of rapamycin (mTOR) /S6K signaling pathway, is activated in the liver of model animals of obesity [12].