ATR and cancer: As the synthetic lethal relationship between Wee1 inactivation and loss of Chk1 is conserved in mammalian cells (Chila et al., 2015), and because inhibitors to human WEE1, ATR and CHK1 have been developed with the aim of targeting cancer cells (Dobbelstein and Sorensen, 2015; Sørensen and Syljuåsen, 2012), understanding the mechanism by which their inactivation leads to cell death is of clinical significance.