ATR is a key regulator of genome integrity, working alongside ATM to respond to DNA damage. ATR is essential for replicating cells, while ATM is not. ATR is activated by single-stranded DNA (ssDNA) bound to replication protein A (RPA), often at stalled replication forks. ATR activation requires the 9-1-1 checkpoint complex and TOPBP1, which recruits ATR to DNA damage sites. ATR signaling involves phosphorylation of numerous substrates, including CHK1, to regulate DNA replication, cell cycle transitions, and DNA repair. ATR and ATM have overlapping but non-redundant functions, with ATR responding to replication stress and ATM to double-strand breaks. ATR is crucial for maintaining genome stability, as its loss leads to severe defects in replication fork stability and increased DNA damage. ATR signaling is a promising target for cancer therapy. ATR is essential for cell viability, particularly in replicating cells, and its dysfunction can lead to genomic instability and disease. ATR's role in DNA repair and checkpoint activation is critical for preventing cancer and maintaining genome integrity. Understanding ATR signaling is important for developing new cancer treatments and therapies targeting DNA damage response pathways.ATR is a key regulator of genome integrity, working alongside ATM to respond to DNA damage. ATR is essential for replicating cells, while ATM is not. ATR is activated by single-stranded DNA (ssDNA) bound to replication protein A (RPA), often at stalled replication forks. ATR activation requires the 9-1-1 checkpoint complex and TOPBP1, which recruits ATR to DNA damage sites. ATR signaling involves phosphorylation of numerous substrates, including CHK1, to regulate DNA replication, cell cycle transitions, and DNA repair. ATR and ATM have overlapping but non-redundant functions, with ATR responding to replication stress and ATM to double-strand breaks. ATR is crucial for maintaining genome stability, as its loss leads to severe defects in replication fork stability and increased DNA damage. ATR signaling is a promising target for cancer therapy. ATR is essential for cell viability, particularly in replicating cells, and its dysfunction can lead to genomic instability and disease. ATR's role in DNA repair and checkpoint activation is critical for preventing cancer and maintaining genome integrity. Understanding ATR signaling is important for developing new cancer treatments and therapies targeting DNA damage response pathways.