To search for proteins whose EV levels correlate with breast tumor trastuzumab response, we generated a trastuzumab-resistant variant of ErbB2-positive breast cancer cells BT-474 [18] (BT-474TR) by exposing the cells in three-dimensional (3D) culture (when cells were detached from the extracellular matrix (ECM) [19]) to trastuzumab and expanding the surviving cells in the presence of the drug [19]. The gene discussed is ERBB2; the disease is breast cancer.