Using the information discovered through yeast cell cycle studies, significant progress has been made regarding the mammalian cell cycle. It has been determined that the cell cycles are similar and CDKCs, either directly or indirectly, affect the progression of the cell cycle. As previously mentioned, in yeast, only one cyclin-dependent kinase (CDK) is associated with several different cyclins. However, in mammalian cells, several different CDKs bind to various cyclins to form CDKCs. For instance, Cdk1 (also known as human Cdc2), the first human CDK to be identified, associates with cyclins A or B. CyclinA/B-Cdk1 complexes drive the transition between G2 phase and M phase, as well as early M phase. Another mammalian CDK, Cdk2, can form complexes with cyclins D1, D2, D3, E, or A. Cdk4 and Cdk6 interact with cyclins D1, D2, and D3. Studies have indicated that there is no difference between CDKCs cyclin D1-Cdk4/6, therefore, any unique properties can possibly be linked to substrate specificity or activation. While levels of CDKs remain fairly constant throughout the cell cycle, cyclin levels fluctuate. The fluctuation controls the activation of the cyclin-CDK complexes and ultimately the progression throughout the cycle. See Table 2 for a summary of mammalian cell CDKCs involved in the cell cycle.