MX1 and breast cancer: Our data tie together previous reports showing that CAFs are associated with poor outcomes in TNBC patients,8 that breast CAFs can secrete IFNβ thereby activating IFN signalling and influencing behaviour of breast cancer cells in vitro,32,33 and that expression of MX1 in breast cancer cells is significantly associated with poor outcomes in patients.34 Critically, we define the functional impact of this signalling on cancer cells in terms of chemotherapy resistance (Figs. 1 and 4), and indeed chemotherapy-treatment itself contributes to induction of full paracrine activity (Figs. 2 and 3;33).