As the telencephalon first emerges from anterior neuroectoderm, signalling molecules and transcription factors combine to specify the identity and fate of cells in each of its regions. Studies of both naturally occurring and transgenic mutant mice have identified many genes that contribute to this process. The development of telencephalon and its regions is specified by signalling molecules produced at sites both surrounding and within the telencephalon. Different parts of the telencephalon express different combinations of transcription factors that control processes including proliferation, cell fate determination and migration in order to create the unique phenotype of each region.