An intrinsic S/G2 checkpoint enforced by ATR.
Saldivar JC, Hamperl S, Bocek MJ, Chung M, Bass TE, Cisneros-Soberanis F, Samejima K, Xie L, Paulson JR, Earnshaw WC, Cortez D, Meyer T, Cimprich KA. Science. 2018 Aug 24;361(6404):806-810. doi: 10.1126/science.aap9346. [PubMed]