A protein functional network describes the functional and physical associations among proteins and provides a framework for understanding how individual protein work together to perform critical cellular functions and how protein compositions respond to changes in cellular environments [6] . Proteins rarely act alone but rather interact with other proteins and comprise specific functional modules in the network [7] . Functional module is a group of proteins which are related by one or more cellular and genetic interactions such as co-regulation, co-expression, and the member of a biological pathway or a protein complex [8] . Such module is the building block of cellular organization and carries out unique biological process [9] . Therefore, understanding the modular structure of protein functional network should be useful for characterizing the dynamic organization of cellular systems.