Several genetic and molecular mechanisms linked to HER2-positive and triple-negative BC breast cancers appear to influence BCBM formation on several levels, including increased development of circulating tumor cells (CTCs), enhanced epithelial-mesenchymal transition (EMT), and migration of primary BC cells to the brain and/or through superior local invasiveness aided by cancer stem-like cells (CSCs). The gene discussed is ERBB2; the disease is cancer.