Understanding the process of cell fate assignment requires us to describe the integration of both intrinsic and extrinsic factors in GRNs, as well as the mechanisms that underlie their precise and reproducible operation ( Davidson,2006). The extrinsic factors principally comprise signals that regulate and coordinate those states and the transitions that occur between them in groups of, and in individual, cells. The intrinsic factors are the transcription factors that a cell expresses at a given time that provide the coordinates that define a cell's state ( Alon, 2006). These networks determine the phenotype of individual cells by providing diverse patterns of gene expression through the activity of intrinsic and extrinsic factors. Here, we shall focus on the second process, which is often referred to as `cell fate specification', and on its driving force, a combination of complex gene regulatory networks (GRNs)( Davidson, 2006). The development of a multicellular organism requires the coordination in space and time of three events: cellular proliferation, the assignment of different fates to an ensemble of cells, and their organization into tissues and organs.