Cyclin A plays a crucial role in regulating the human cell cycle, particularly in S phase and G2. The authors found that microinjection of anti-cyclin A antibodies at different stages of the cell cycle inhibited DNA synthesis and mitosis. Cyclin A binds to both cdc2 and cdk2, forming two distinct kinases with different activities. The kinase activity associated with cdc2 appears in S phase, while the activity with cdk2 appears in G2. These findings suggest that cyclin A defines novel control points in the human cell cycle. The study also explored the subcellular distribution of cyclin A and its interaction with cdc2 and cdk2, providing insights into the regulation of the cell cycle by cyclin A.Cyclin A plays a crucial role in regulating the human cell cycle, particularly in S phase and G2. The authors found that microinjection of anti-cyclin A antibodies at different stages of the cell cycle inhibited DNA synthesis and mitosis. Cyclin A binds to both cdc2 and cdk2, forming two distinct kinases with different activities. The kinase activity associated with cdc2 appears in S phase, while the activity with cdk2 appears in G2. These findings suggest that cyclin A defines novel control points in the human cell cycle. The study also explored the subcellular distribution of cyclin A and its interaction with cdc2 and cdk2, providing insights into the regulation of the cell cycle by cyclin A.