Figure 1: Structure (A) and activation (B) of a typical tyrosine kinase receptor, the receptor binding epidermal growth factor (EGF). These receptors have one transmembrane segment. The extracellular portion of the receptor binds the ligand (EGF in this case). Inside the cell, a portion of the receptor has tyrosine kinase activity. The remainder of the receptor contains a series of tyrosine residues that are substrates for the tyrosine kinase. The activation of receptor tyrosine kinases starts with the binding of a messenger, causing receptor aggregation or clustering. Once the receptors aggregate, they cross-phosphorylate each other at a number of tyrosine amino acid residues (C). The formation of tyrosine phosphate (Tyr-P) residues on the receptor creates binding sites for cytosolic SH2 domains.