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 Cyclin–Cdk complexes of the cell-cycle control system. The concentrations of 
the three major cyclin types oscillate during the cell cycle, while the concentrations of Cdks (not 
shown) exceed cyclin amounts and do not change. In late G1, rising G1/S-cyclin levels lead to 
the formation of G1/S-Cdk complexes that trigger progression through the Start transition. S-Cdk 
complexes form later in G1 and trigger DNA replication, as well as some early mitotic events. 
M-Cdk complexes form during G2 but are held in an inactive state; they are activated at the end of 
G2 and trigger entry into mitosis at the G2/M transition. A separate regulatory protein complex, the 
APC/C, initiates the metaphase-to-anaphase transition, as we discuss later.