[
2003]
During development and homeostasis, individual cells must divide, differentiate, migrate, adapt to the environment, or die at the appropriate times and places. A key to deciphering the molecular mechanisms by which cells make these decisions is to characterize the regulation and function of the proteins that regulate important changes in gene expression. The family of transcription factors that contain basic-helix-loop-helix and PAS motifs has been shown to control many critical developmental events and to mediate responses to certain environmental stimuli. For example, bHLH-PAS proteins play central roles in the development of specific neural tissues and vasculature, and they are core components of the molecular clock that govern circadian rhythms. bHLH-PAS proteins are also integral to the pathways that sense and respond to hypoxia (low oxygen) and certain xenobiotics (1). Phylogenetic analyses suggest that bHLH-PAS genes arose early in animal development, and in some cases, the functions of individual genes are largely conserved across phyla. This review describes the bHLH-PAS gene family in a genetic model organism, the nematode Caenorhabditis elegans.