The study by Eric J. Brown and David Baltimore investigates the role of ATR (ataxia telangiectasia and radiation-sensitive protein) in early embryonic development. ATR disruption leads to early embryonic lethality, with embryos dying after the blastocyst stage and before 7.5 days post-coitum (p.c.). In vitro culture experiments show that ATR−/− blastocyst cells continue mitosis for 2 days but then fail to expand and undergo caspase-dependent apoptosis. Chromosome breaks are observed in ATR−/− cells before widespread apoptosis, suggesting that apoptosis is caused by a loss of genomic integrity. The data indicate that ATR is essential for early embryonic development and functions in processes other than p53 regulation. The authors discuss the possibility that ATR may regulate BRCA gene products and control the S to M phase transition in early embryonic cells.The study by Eric J. Brown and David Baltimore investigates the role of ATR (ataxia telangiectasia and radiation-sensitive protein) in early embryonic development. ATR disruption leads to early embryonic lethality, with embryos dying after the blastocyst stage and before 7.5 days post-coitum (p.c.). In vitro culture experiments show that ATR−/− blastocyst cells continue mitosis for 2 days but then fail to expand and undergo caspase-dependent apoptosis. Chromosome breaks are observed in ATR−/− cells before widespread apoptosis, suggesting that apoptosis is caused by a loss of genomic integrity. The data indicate that ATR is essential for early embryonic development and functions in processes other than p53 regulation. The authors discuss the possibility that ATR may regulate BRCA gene products and control the S to M phase transition in early embryonic cells.