Protein-protein or protein-ligand  interactions are essential to almost every process in a cell, so understanding these interactions is crucial for understanding cell physiology in normal and disease states. It is also essential in drug development, since drugs can affect protein-protein or protein-ligand interaction. Protein interaction  is mathematical representations of the physical contacts between proteins or protein-ligand in the cell. These contacts:

• are specific;
• occur between defined binding regions in the proteins or protein-ligand