**4.1 Adapter proteins**

Adapter proteins do not exert any kinase activity, but they regulate protein – protein interaction and help the formation of protein complex which participate in signal transduction pathways. GRB2-associated-binding protein 2 (Gab2) is one of the adapter proteins which is overexpressed in breast cancer. It promotes signaling pathways by recruiting SH2 containing proteins such as PI3K, Shc, and Shp2 downstream of tyrosine kinase receptors. Although elevated expression of Gab2 in the mammary epithelium is unable to induce tumor development, it has been shown that tumor onset time will decrease in presence of Gab2 (16,17)
