Cyclin-dependent kinases (Cdks) and their regulatory cyclins are crucial for cell cycle progression, but recent studies challenge the notion that they are strictly essential. In mice, genes encoding D-type and E-type cyclins, along with their associated Cdks, are not strictly required for fetal development, suggesting redundancy in cell cycle regulation. While Cdks and cyclins are vital for G1 phase progression and DNA replication, their absence does not always lead to developmental defects, indicating that other pathways can compensate. In yeast, G1 cyclins and Cdks are essential for cell cycle progression, but in mammals, the role of cyclins and Cdks is more nuanced. D-type cyclins (D1, D2, D3) and their partners Cdk4 and Cdk6 are involved in early G1 phase, while E-type cyclins (E1, E2) and Cdk2 are critical for later G1 and S phase. However, some cyclins and Cdks, like cyclin D and Cdk4, are not essential for viability in Drosophila, suggesting that their functions can be substituted by other regulators. Cdk2 is essential for S phase in Drosophila and Xenopus, but its role in mammals is more complex. While Cdk2 is necessary for S phase, its activity can be compensated by other kinases, such as cyclin E-Cdk2, which also plays a role in G1 phase. Recent studies show that Cdk2 is not strictly essential for cell cycle progression, as Cdk2-null mice survive with some defects, indicating that other kinases can compensate. Cyclin E is essential for loading the MCM helicase onto chromosomal DNA, which is necessary for DNA replication. However, cyclin E-null mice exhibit some defects, but not all, suggesting that other pathways can compensate. The loss of cyclin E does not always lead to embryonic lethality, indicating that other regulators can take over its functions. In cancer, cyclin D and E-dependent kinases are not strictly essential for cell cycle progression, but they play critical roles in exiting quiescence. MEFs lacking cyclin D or E show resistance to oncogenic transformation, suggesting that these kinases are important for cell cycle entry and response to mitogenic signals. However, the loss of cyclin D or E does not always lead to cancer resistance, indicating that other factors may be involved. Overall, while cyclins and Cdks are important for cell cycle regulation, their absence does not always lead to developmental defects, suggesting that other pathways can compensate. This highlights the complexity of cell cycle regulation and the importance of redundancy in maintaining normal development and preventing cancer.Cyclin-dependent kinases (Cdks) and their regulatory cyclins are crucial for cell cycle progression, but recent studies challenge the notion that they are strictly essential. In mice, genes encoding D-type and E-type cyclins, along with their associated Cdks, are not strictly required for fetal development, suggesting redundancy in cell cycle regulation. While Cdks and cyclins are vital for G1 phase progression and DNA replication, their absence does not always lead to developmental defects, indicating that other pathways can compensate. In yeast, G1 cyclins and Cdks are essential for cell cycle progression, but in mammals, the role of cyclins and Cdks is more nuanced. D-type cyclins (D1, D2, D3) and their partners Cdk4 and Cdk6 are involved in early G1 phase, while E-type cyclins (E1, E2) and Cdk2 are critical for later G1 and S phase. However, some cyclins and Cdks, like cyclin D and Cdk4, are not essential for viability in Drosophila, suggesting that their functions can be substituted by other regulators. Cdk2 is essential for S phase in Drosophila and Xenopus, but its role in mammals is more complex. While Cdk2 is necessary for S phase, its activity can be compensated by other kinases, such as cyclin E-Cdk2, which also plays a role in G1 phase. Recent studies show that Cdk2 is not strictly essential for cell cycle progression, as Cdk2-null mice survive with some defects, indicating that other kinases can compensate. Cyclin E is essential for loading the MCM helicase onto chromosomal DNA, which is necessary for DNA replication. However, cyclin E-null mice exhibit some defects, but not all, suggesting that other pathways can compensate. The loss of cyclin E does not always lead to embryonic lethality, indicating that other regulators can take over its functions. In cancer, cyclin D and E-dependent kinases are not strictly essential for cell cycle progression, but they play critical roles in exiting quiescence. MEFs lacking cyclin D or E show resistance to oncogenic transformation, suggesting that these kinases are important for cell cycle entry and response to mitogenic signals. However, the loss of cyclin D or E does not always lead to cancer resistance, indicating that other factors may be involved. Overall, while cyclins and Cdks are important for cell cycle regulation, their absence does not always lead to developmental defects, suggesting that other pathways can compensate. This highlights the complexity of cell cycle regulation and the importance of redundancy in maintaining normal development and preventing cancer.