Apolipoprotein CIII (apoCIII) plays an important role in plasma triglyceride and remnant lipoprotein metabolism. Because hypertriglyceridemia is an independent risk factor in coronary artery disease and the presence in plasma of triglyceride-rich remnant lipoproteins is correlated with atherosclerosis, considerable research efforts have been focused on the identification of factors regulating apoCIII gene expression to decrease its production. Here we report that the orphan nuclear hormone receptor Rev-erb regulates the human apoCIII gene promoter. In apoCIII expressing human hepatic HepG2 cells, transfection of Rev-erb specifically repressed apoCIII gene promoter activity. We determined by deletion and sitedirected mutagenesis experiments that Rev-erb dependent repression is mainly due to an element present in the proximal promoter of the apoCIII gene. In contrast, we found no functional Rev-erb response elements in the convergently transcribed human apoAI gene or the common regulatory enhancer. The identified Rev-erb response element coincides with a ROR 1 element, and in the present study we provide evidence that functional cross-talk between these orphan receptors modulates the apoCIII promoter. In vitro binding analysis showed that monomers of Rev-erb bound this element but not another upstream ROR 1 response element. In addition, we showed that the closely related nuclear orphan receptor RVR also specifically repressed the human apoCIII gene. These studies underscore a novel physiological role for members of the Rev-erb family of nuclear receptors in the regulation of genes involved in triglyceride metabolism and the pathogenesis of atherosclerosis.