The human nervous system contains billions of cells and thousands of cell types. Cells differ from each other in many ways, including in their shape and in the type of neurotransmitter they express. Each type of cell furthermore needs to be generated at the correct place, correct time, and in proper numbers.

The combined effect of great numbers, great diversity and great fidelity constitutes the very basis for the enormously complex functions of the nervous system, such as homeostasis, learning/memory and behavior. However, due to the complexity of the nervous system we still have major knowledge gaps in our understanding of how different cell types are specified and how proliferation is controlled.

Resolving these fundamental issues will likely be important for our understanding not only of normal development, but also of various neurological disorders and cancer, since there is increasing awareness that many such diseases may be caused by dysfunctional developmental programs, including proliferation control.