NTRK2 and neoplasm: Using brain-tropic as well as non-selected models of TNBC, we have defined a novel mechanism by which E2-dependent upregulation of brain-derived neurotrophic factor (BDNF) in astrocytes, and subsequent activation of tumor cell tropomyosin kinase receptor B (TrkB) in TNBC cells, promotes brain metastatic colonization.