Although it was known that inhibition of ATR, CHK1, and WEE1 reduces dNTP concentrations in cancer cells (Pfister et al., 2015), our findings that ATR kinase activity is essential to limit dU contamination and that dT partially rescues ATRi-induced CD8+ T cell and cancer death are, in our view, unanticipated. Here, CHEK1 is linked to cancer.