first cyclin box found in cyclin-O (CCNO) and similar proteins
CCNO is specifically required for generation of multiciliated cells, possibly by promoting a cell cycle state compatible with centriole amplification and maturation. It acts downstream of MCIDAS (multiciliate differentiation and DNA synthesis associated cell cycle protein) to promote mother centriole amplification and maturation in preparation for apical docking. CCNO is involved in the activation of cyclin-dependent kinase 2. CCNO contains two cyclin boxes. This model corresponds to the first one. The cyclin box is a protein binding domain.