Increased levels of Mcl-1 (myeloid cell factor-1) have been reported in several cancers, suggesting an important role played by Mcl-1 in cancer cell survival. Mcl-1 is an anti-apoptotic protein shown to delay or block apoptosis. In this work, using semiquantitative immunofluorescence, real-time PCR, and RNase protection assay, an increase in Mcl-1 expression was detected in hepatoma HepG2 cells incubated under hypoxia or in the presence of cobalt chloride. Through analysis of the Mcl-1 promoter sequence, a putative HIF-1 (hypoxiainducible factor-1) binding site was identified. A Mcl-1 promoter fragment containing this hypoxia-responsive element was able to bind HIF-1 in vitro. It also induced hypoxia-dependent transcription of a luciferase reporter gene, which was suppressed by anti-HIF-1alpha short interfering RNA. Finally, overexpression of Mcl-1 protected HepG2 cells against apoptosis induced by tert-butyl hydroperoxide as shown by inhibition of caspase-3 activation and DNA fragmentation. All these data suggest a potential anti-apoptotic role of HIF-1 that could protect cells against apoptosis under hypoxia by overexpression of the Mcl-1 protein.