[The vertebrate nervous system comprises an enormous number of cell types].

Neurogenesis is regulated by basic-helix-loop-helix (bHLH) transcription factors in both vertebrates and insects. MASH1 and neurogenin, members of the bHLH genes, are expressed in subsets of neural precursors and control their differentiation. Downstream of the bHLH genes, PHD1, Phox2, and DRG11, which belong to PHD family genes, are expressed in specific lineages of neurons and are involved in their neuronal identity. Differentiated sensory and motor neurons express PEA3 and ER81, members, of the ETS family, which specify neuronal connectivity. During neuronal differentiation, these three families of transcription factors control developmentally distinct operations. Increasing the numbers of members of the families may underlie the generation of neuronal diversity.