Apart from the expected downregulation of hormone receptors and upregulation of Ki-67 in all tumor subtypes of MT compared to PT, ERBB2 upregulation was observed in MT samples from triple-negative PT patients (p = 0.292), whereas ERBB2 overexpression was particularly significant in brain MTs in initially triple-negative PT BC (p = 0.023). Here, ERBB2 is linked to breast cancer.