In the developing mammalian nervous system, neural progenitor cells first 
express the Notch effector Hes1 at variable levels and then proneural genes and 
Notch ligands in salt-and-pepper patterns. Recent real-time imaging analysis 
indicates that Hes1 expression in these cells oscillates with a period of about 
2-3 h. Furthermore, the proneural gene Neurogenin-2 (Ngn2) and the Notch ligand 
gene Deltalike-1 (Dll1) are expressed cyclically in neural progenitor cells 
under the control of Hes1 oscillation but are expressed continuously in 
postmitotic neurons, which lose Hes1 expression. Hes1-driven Ngn2 and Dll1 
oscillations seem to be advantageous for maintenance of a group of cells in an 
undifferentiated state by mutual activation of Notch signaling. This dynamic 
mode of gene expression would require a revision of the traditional view of how 
Notch-mediated lateral inhibition operates in the developing mammalian nervous 
system.