first cyclin box found in cyclin-K (CCNK) and similar proteins
CCNK is a novel RNA polymerase II-associated C-type cyclin possessing both carboxy-terminal domain kinase and Cdk-activating kinase activity. It is a regulatory subunit of cyclin-dependent kinases that mediates the activation of these target kinases. It plays a role in transcriptional regulation by controlling the phosphorylation of the C-terminal domain (CTD) of the large subunit of RNA polymerase II (POLR2A). CCNK contains two cyclin boxes. This model corresponds to the first one. The cyclin box is a protein binding domain.