Conjugated with a releasable cell-penetrating peptide tag, Transactivator of Transcription (TAT) peptide, Cyclo-C(TAT)LLFVY demonstrated selective binding to the PAS-B domain of HIF-1α, exhibiting a KD of 124 nM by ITC and effectively inhibited HIF-1 signaling in hypoxic MCF-7 and U2OS cancer cells. This evidence concerns the gene HIF1A and cancer.