A functional binding site for the transcriptional regulator hypoxia-inducible factors (HIFs) has been identified on the CD55 promoter.20 HIFs (HIF-1α, HIF-2α) have been shown to be regulators of the response to low oxygen concentration in various solid tumors and to be related to tumor malignancy.28 The well-known CD55 expression upon HIF stabilization in hypoxia and CD55 expression in solid tumors suggest CD55 as a novel HIF target that contributes to tumor defense mechanisms for survival in adverse physiological conditions. This evidence concerns the gene EPAS1 and neoplasm.