BMP9 potentiates early endothelial cell response to hypoxia. HMVEC-D microvascular cells were incubated under standard normoxic (5% CO₂, 95% air) or hypoxic conditions achieved by repeated purging with a mixture of 5% CO₂, 1% O₂, and 94% N₂. Cells were treated with 5 ng/mL BMP9 as indicated, and quantitative RT-PCR was used to quantify endoglin and SDF1 mRNA levels. The normalizing control was normoxia in the absence of added BMP9. Error bars represent SEM for duplicate experiments. Establishment of hypoxia was independently judged effective by staining cells with the oxygen-sensing dye pimonidazole (Hypoxyprobe⁵¹ ). The dye, which is not detectable under normoxia, becomes fluorescent in at levels of oxygen approaching 1% (arrow, inset panels). *P < .05.