ATR is an attractive target because cancer cells with inactivating mutations in ARID1A, ATM, ERCC1, RNASE H2, and XRCC1 are sensitive to ATRi’s in vitro and in vivo, particularly in combination with DNA-damaging agents that target DNA replication forks (Hustedt et al., 2019; Mohni et al., 2014, 2015; Vendetti et al., 2015; Wang et al., 2019; Williamson et al., 2016). Here, ATR is linked to cancer.