In fact, several studies have shown that basal and inducible AHR activity is repressed under hypoxic conditions,72,73 and mechanistic studies on rodent hepatoma cell lines revealed ARNT as the limiting factor.73,74 However, in subsequent studies on human cell lines75 and murine HIF-1α-null hepatocytes,76 the competition of both transcription factors for ARNT could not be confirmed. This evidence concerns the gene ARNT and hepatocellular carcinoma.