CDK9 and breast carcinoma: DNA-PK assembles with hypoxia-inducible factor 1 (HIF-1) and tripartite motif containing protein 28 (TRIM28) to form heterotrimers, which stabilize the HIF occupancy of hypoxia response elements (HREs) and recruit cyclin-dependent kinase 9 (CDK9) to phosphorylate Pol II C-terminal domain (CTD) on Ser2, leading to HIF transcriptional activation in hypoxic breast cancer cells [7].