IRS1 (Insulin Receptor Substrate 1) transmits insulin signals via metabolic and mitogenic pathways. IRS1 is heavily phosphorylated on both serine and tyrosine residues. These phosphorylated tyrosines enable IRS to act as a docking protein that binds SH2 domains of such proteins as PI3 Kinase (phosphatidylinositol 3-kinase) and GRB2, resulting in activation. Over expression and phosphorylation of serine is associated with insulin resistance and breast cancer. Some of the more notable phosphorylation sites are Ser302 that is phosphorylated following insulin stimulation. Ser307, phosphorylated by JNK and IKK, is a key regulatory site that appears to disrupt the IRS1/IR interaction and inhibits insulin-mediated activation of the PI3 kinase and MAPK pathways, and Ser636/639 that is known to be phosphorylated by p70S6K downstream of mTOR and acts as a negative feedback loop.