Cell cycle checkpoints ensure accurate DNA replication and repair, preventing the transmission of genetic damage to daughter cells. These checkpoints are activated by DNA damage, such as DNA strand breaks or replication errors, and coordinate cell cycle progression with DNA repair. In certain cell types, checkpoint proteins link DNA damage to apoptosis. ATM and ATR kinases are key players in checkpoint signaling, detecting DNA damage and initiating repair. The Rad family of checkpoint proteins, including Rad1, Rad9, and Hus1, form a complex that may function as a checkpoint sliding clamp, facilitating the recruitment of checkpoint signaling machinery to damaged DNA. The PIKK family, including ATM and ATR, is crucial for checkpoint signaling, with ATM and ATR playing central roles in early signal transmission. These kinases phosphorylate target proteins, such as p53, to regulate cell cycle arrest and DNA repair. ATM and ATR also interact with other proteins, such as hChk2, to modulate p53 activity and stabilize the protein. The activation of ATM and ATR in response to DNA damage involves complex signaling pathways, with ATM primarily responding to ionizing radiation (IR) and ATR to other forms of genotoxic stress. The functions of ATM and ATR in checkpoint signaling are distinct, with ATM being more involved in the G1 checkpoint and ATR in the S and G2 checkpoints. The identification of ATM and ATR substrates has revealed their roles in regulating cell cycle progression and DNA repair. The mechanisms by which ATM and ATR detect and respond to DNA damage are still being elucidated, with ongoing research aiming to understand their roles in maintaining genomic stability.Cell cycle checkpoints ensure accurate DNA replication and repair, preventing the transmission of genetic damage to daughter cells. These checkpoints are activated by DNA damage, such as DNA strand breaks or replication errors, and coordinate cell cycle progression with DNA repair. In certain cell types, checkpoint proteins link DNA damage to apoptosis. ATM and ATR kinases are key players in checkpoint signaling, detecting DNA damage and initiating repair. The Rad family of checkpoint proteins, including Rad1, Rad9, and Hus1, form a complex that may function as a checkpoint sliding clamp, facilitating the recruitment of checkpoint signaling machinery to damaged DNA. The PIKK family, including ATM and ATR, is crucial for checkpoint signaling, with ATM and ATR playing central roles in early signal transmission. These kinases phosphorylate target proteins, such as p53, to regulate cell cycle arrest and DNA repair. ATM and ATR also interact with other proteins, such as hChk2, to modulate p53 activity and stabilize the protein. The activation of ATM and ATR in response to DNA damage involves complex signaling pathways, with ATM primarily responding to ionizing radiation (IR) and ATR to other forms of genotoxic stress. The functions of ATM and ATR in checkpoint signaling are distinct, with ATM being more involved in the G1 checkpoint and ATR in the S and G2 checkpoints. The identification of ATM and ATR substrates has revealed their roles in regulating cell cycle progression and DNA repair. The mechanisms by which ATM and ATR detect and respond to DNA damage are still being elucidated, with ongoing research aiming to understand their roles in maintaining genomic stability.