IRS1 and obesity due to melanocortin 4 receptor deficiency: While Src and Ras/Raf/MEK/ERK signaling represent the best characterized pathways used during tumor development, several additional PTP1B effectors such as the IR, IRS-1, and JAKs/STATs have been implicated in type 2 diabetes, obesity, and interestingly in the modulation of immune response, thus representing a very attractive and complicated therapeutic target.