Characterization of P4Hα1 promoter responsible for TNF-α–mediated suppression. We transfected HASMCs with 9 serially deleted P4Hα1 promoter-pGL3 constructs: pGL3–580, pGL3–480, pGL3–417, pGL3–320, pGL3–271, pGL3–184, pGL3–97, pGL3–32, and pGL3-+18. PGL-3 basic vector was used as a control. The transfected cells were treated with or without 100 ng/mL TNF-α for 8 hours. Quantitative real-time RT-PCR was used to measure the levels of luciferase mRNA. **P<0.01 is result of the Student t test between cells treated with and without TNF-α.