The PH domain is a protein module of approximately 120 amino acid residues founded in many proteins involved in signal transduction. The PH domains are similar to each other in their three-dimentional structures,and the major structure difference among them lies in the three variable loops in the structures. The PH domain is electrostatically-polarized and the variable loops are on the positively-charged surface, which may serve as a ligand-binding surface. So far ,it has been found that PH domains can interact with the βγ-subunits of G protein (Gβγ)、protein kinase C (PKC) and phosphatidylinositol-4,5-bisphosphate (PIP₂ or inositol-1,4,5-trisphosphate (IP₃)). All these implied that PH domain might play an important role in the interaction between the signaling molecules and help to form the signal transduction network.