Using human ER+ BC specimens analyzed by immunofluorescence microscopy, we found the existence of Notch1+ BC cells within the microvasculature, suggesting a direct interaction with arteriolar ECs, which was supported by a vascular-like structure consisting of DLL4+ and DLL4+/Notch1+ cells and blood vessel-like structure composed of DLL-4+ cells (Supplementary Fig. 4). Here, ESR1 is linked to breast cancer.