To verify our hypothesis, we used highly metastatic human breast cancer cells (Hs578T and MDA-MB-231) in which endogenous TrkB expression was depleted by short hairpin RNA (shRNA) using a lentivirus system [16] to explore whether loss of TrkB modulates TGF-β signaling in highly metastatic breast cancer cells (Figure S2A). The gene discussed is TGFB1; the disease is breast cancer.