As a test for indirect crosstalk, we used transwell chambers to coculture estrogen receptor-positive (ER+) BC cells (MCF-7) and human microvascular ECs (HMVECs) that express delta-like ligand 4 (DLL-4), a marker of well-differentiated arteriolar ECs and a ligand involved with Notch1 signaling. Here, DLL4 is linked to breast cancer.