The C-terminal domain kinase (CTDK-1), is a three-subunit complex comprised of Ctk1, Ctk2, and Ctk3, that plays a key role in regulation of transcription and translation and in coordinating these two processes. Both Ctk2 and Ctk3 are regulated at the level of protein turnover, and are unstable proteins processed through a ubiquitin-proteasome pathway. Their physical interaction is required to protect both subunits from degradation, and both Ctk2 and Ctk3 are required for Ctk1 CTD kinase activation. The mammalian P-TEFb is mirrored by the combined complexes in yeast of the CTDK1 and the Bur1/2. It is not clear what independent function this C-terminal domain has.