Since this method does not induce metastasis to any other organs but to the central nervous system and the same number of tumor cells are injected into the same brain region with this method, we employed the intracranial injection method to evaluate the brain-colonizing ability of HER2-positive breast cancer cells to elucidate the mechanism of the later stages of brain metastasis. The gene discussed is ERBB2; the disease is breast carcinoma.