PER2 and metabolic disease: This suggests that heat shock factors may mediate the rhythmicity of Per2 and Rev-erbα in the fetus, particularly given the presence of heat shock response elements in the promoter of at least Per2. It cannot be dismissed that disruptions to the timing of circadian gene expression within the developing fetus, as observed here in the fetal liver, may play some role in the programming of adult metabolic disease.