sentences:  Once single-stranded DNA becomes sufficiently long, single-stranded DNA coated with RPA are able to recruit ATR-ATRIP.In order to become fully active, the ATR kinase rely on sensor proteins that sense whether the checkpoint proteins are localized to a valid site of DNA replication stress.The RAD9-HUS1-Rad1 (9-1-1) heterotrimeric clamp and its clamp loader RFCRad17 are able to recognize gapped or nicked DNA.