Clocked gene expression in somite formation.

A recent paper describes a striking expression pattern during somite formation for a chick ortholog of the fly hairy gene. Before segmentation, c-hairy1 mRNA oscillates in the presomitic mesoderm such that three distinct spatial patterns are seen. The authors use a series of ingenious manipulations to show that these phases follow each other in time, adding up to a 90-minute periodicity in c-hairy1 expression. The discovery of this clock of gene expression emphasizes the importance of temporally regulated events in the establishment of spatial patterns.