Negatively regulates endocycles and acts as a regulator of ploidy levels in endoreduplication (PubMed:16415207). Promotes divisions in the guard cells (GCs) after the guard mother cells (GMC) symmetric division (PubMed:24687979). Interacts with CDKA-1 (PubMed:16415207). Interacts with SAMBA (PubMed:22869741). Repressed by MYB88 and MYB124 in newly formed guard cells. Belongs to the cyclin family. Cyclin AB subfamily.