Cells were cultured as detailed in the Online Supplementary Appendix. Briefly, 4x105, 3.5x105 and 6.5x105 cells were transiently transfected with 500 ng or 100 ng reporter plasmid in a 6-well format using CaCl2 or JetOptimus (Polyplus), respectively for HEK293T and Hep3B or Kelly cells, and 1 mg of YFP-HIF-1α, YFP-HIF-2α or pcDNA3-HA-HIF-2α (Add-gene) constructions. In order to control for differences in transfection efficiency and extract preparation, 50 ng or 75 ng pRL-SV40 Renilla luciferase reporter vector (Promega) was co-transfected, respectively for HEK293T and Hep3B or Kelly cells. The next day, cultures were evenly split onto 6-well plates, incubated for an additional 24 hours, under normoxic or hypoxic conditions (0.2% O2, 5% CO2 and 37°C). Cells were lysed with passive lysis buffer and luciferase activities of duplicated wells were determined using the Dual Luciferase Reporter Assay System (Promega) as described before.24 (link) Reporter activities were expressed as relative Firefly/Renilla luciferase activities normalized to control under hypoxic conditions. All reporter gene assays were performed at least three times independently. Proteins were extracted and immunoblotted to quantify the HIF-2α proteins as described before25 (link) (see the Online Supplementary Appendix).