English: In animals (for example, H. sapiens, left), a trimeric CAK enzyme containing Cdk7 functions both in the activation of Cdks and in the regulation of transcription by RNA polymerase II. In the budding yeast S. cerevisiae (right) the homologous enzyme, Kin28, does not contribute to Cdk activation but is focused entirely on control of trans- cription. In this species, an unrelated protein kinase, Cak1, activates Cdks. The fission yeast S. pombe (center) occupies an intermediate position, in which Cdk activation can be achieved both by the Cdk7 homolog Mcs6 and by a Cak1 homolog, Csk1. Cdk7, Kin28 and Mcs6 are all Cdks whose activities are also enhanced by phosphorylation of residues in their T-loops. In budding and fission yeasts, this phosphorylation is carried out by Cak1 and Csk1, respectively. The kinase that phosphorylates Cdk7 in animals is not clear.^[1]