An overview of CDK functions in the cell. Each CDK (in orange boxes) is shown in a complex with its major partner (green) - for clarity, only a few substrates are depicted. Most CDKs function in the nucleus (orange background), whereas a few family members are attached to the cell membrane or display cytoplasmic activities (blue background). Classical cell cycle CDKs - Cdk4, Cdk6, Cdk2 and Cdk1 - regulate the transitions through the different phases of the cell-division cycle. These activities are at least partially mediated by the control of multiple transcription factors (TFs) or regulatory elements such as the retinoblastoma protein (Rb). Cdk10 and Cdk11 also control transcription by phosphorylating TFs, hormone receptors and associated regulators (HRs), or splicing factors (SPFs). Cdk7, Cdk9 and Cdk12 directly phosphorylate the C-terminal domain (CTD) of RNA polymerase II (RNAPII), thus modulating the different phases of generation of transcripts. The Mediator complex is specifically regulated by Cdk8 or the highly related Cdk19. Cdk7 functions as a CDK-activating kinase (CAK) by directly phosphorylating several of the CDKs mentioned above. Cdk5 displays many functions in the cell, but it is better known for its function in the control of neuron-specific proteins such as Tau. The members of the Cdk14 subfamily, such as Cdk14 itself or Cdk16, are activated at the membrane by cyclin Y and also participate in many different pathways, such as Wnt-dependent signaling or signal transduction in the primary cilium. It is important to note that, for clarity, many interactions between CDKs and other partners, substrates or cellular processes are not shown - for instance, Cdk1 can bind to other cyclins and can also phosphorylate more than 100 substrates during mitotic entry that are not indicated here. CAK, CDK-activating kinase; CDK, cyclin-dependent kinase; CTD, C-terminal domain; Rb, retinoblastoma protein; RNAPII, RNA polymerase II; SPF, splicing factor; TF, transcription factor.