first cyclin box found in Saccharomyces cerevisiae CTD kinase subunit 2 (ScCTK2) and similar proteins
CTK2, also called CTD kinase subunit beta, CTDK-I subunit beta, or CTD kinase 38 kDa subunit, is the cyclin subunit of the CTDK-I complex, which hyperphosphorylates the C-terminal heptapeptide repeat domain (CTD) of the largest RNA polymerase II subunit. CTK2 contains two cyclin boxes. This model corresponds to the first one. The cyclin box is a protein binding domain.