Cln1, Cln2, and Cln3 are cyclin proteins expressed in the G1-phase of the cell cycle of budding yeast. Like other cyclins, they function by binding and activating cyclin-dependent kinase. They are responsible for initiating entry into a new mitotic cell cycle at Start. As described below, Cln3 is the primary regulator of this process during normal yeast growth, with the other two G1 cyclins performing their function upon induction by Cln3. However, Cln1 and Cln2 are also directly regulated by pathways sensing extracellular conditions, including the mating pheremone pathway.[1]