Early embryonic territories are transient entities under permanent remodelling to form newly derived cell populations that will eventually give rise to the adult tissues and organs. A vast effort has been devoted to identifying the determinants and mechanisms that define embryonic territories. Indeed, studies in the vertebrate embryo from the morula stage to the segregation of the main embryonic layers-ectoderm, mesoderm and endoderm-have highlighted the importance of the mutual exclusion/repression between pairs of transcription factors, in coordination with the control exerted over cell division, adhesion and motility.