CDK9(Cyclin-dependent kinase 9) is a member of the Cdc2-like family of kinases. Its cyclin partners are members of the family of cyclin T (T1, T2a and T2b) and cyclin K. Phospho-CDK9 (Thr186) is the active-site-switching modification of the catalytic subunit of P-TEFb (positive transcription elongation factor b). Phosphorylation at threonine 186, located in the T-loop of CDK9, is essential for full kinase activity: it swings the T-loop away from the catalytic cleft, allowing ATP and protein substrates (e.g., the C-terminal domain of RNA polymerase II, DSIF and NELF) to enter and be efficiently phosphorylated. Thus, p-Thr186-CDK9 licenses the elongation step of most protein-coding transcripts and is indispensable for productive mRNA synthesis. (PMID: 39800748)