I. Introduction
When performing their functions, proteins seldom do so individually. Instead, they do so by interacting with each other as a whole [1]. If the interactions among proteins can be predicted accurately, we may therefore be able to better discover unknown functions of a protein based on the known functions of those proteins that it interacts with. In addition, we may also be able to better understand the molecular mechanisms of many biological processes such as DNA regulation, cell signaling, assembly of protein complexes, etc. Given the importance, various attempts have been made to develop techniques to effectively predict protein-protein interactions (PPIs).