G1/S-specific cyclin essential for the control of the cell cycle at the G1/S (start) transition and for maintenance of filamentous growth. Through binding to CDC28 controls the phosphorylation of CDC11 and SEC2 upon induction of filamentous growth. Interacts with CDC28. The CDC28-CCN1 complex associates with septin CDC11 upon hyphal induction. Expression peaks at G1/S. CCN1 is stabilized during germ tube formation. Belongs to the cyclin family.