Since HIF-1α and HIF-2α both dimerize with HIF-1β, and recognize a similar consensus DNA sequence, but have opposing effects in tumor xenograft growth in 786–0, we first considered the possibility that re-expressed HIF-1α might antagonize HIF-2α DNA-binding, either through direct competition for binding sites or through competition for HIF-1β. This evidence concerns the gene EPAS1 and neoplasm.