Following breast-conserving surgery and sentinel lymph node biopsy, histopathological and immunohistochemical analysis revealed a collision tumor composed of myeloid sarcoma (MS) and invasive ductal carcinoma (IDC), the latter exhibiting a triple-negative phenotype (ER-, PR-, HER2-), with no lymph node metastasis. The gene discussed is ERBB2; the disease is neoplasm.