Cell signalling is the communication inside a cell or with its environment. It plays an important role in cellular growth, survival and apoptosis. For instance, the growth of a cell requires production of certain proteins. The genes that encode these proteins are located in DNA inside the nucleus while the initial signal for cellular growth may come from outside the cell. To induce the expression of these genes, the external signal must pass through specific pathways to reach these genes in DNA. The signalling pathway involves several proteins. If any one of them becomes abnormal or non-functional, the cellular process would be altered, which may cause disease. Therefore, knowing the detailed pathway is very important in understanding the pathogenesis of diseases.