Evolutionary relationships among the mammalian CDK subfamilies. The name of the different CDK subfamilies functioning in the cell cycle (orange) or transcription (green) is shown in boldface, and the domain structure of the individual proteins is depicted. The conserved protein kinase domain (red) and some additional domains (see key) are indicated for each CDK. Human cells contain two separate genes, Cdk11A and Cdk11B, each of them encoding a long isoform, Cdk11p110, and a shorter protein, Cdk11p58, generated by an internal ribosome binding site. The phylogenetic tree is based on the comparison of the human kinase domains . CDK, cyclin-dependent kinase.