ATR and breast cancer: Taken together, these findings support the concept that the interplay between SLX4IP and TERT dictates TMM selection and drug sensitivity, such that (i) SLX4IPLow/TERTHigh breast cancer cells succumb to 5-FdU treatment and resist ATR inhibition, and (ii) SLX4IPHigh/TERTLow breast cancer cells succumb to ATR inhibition and resist 5-FdU treatment.