ERBB2 and neoplasm: However, when MMTV-HER2 mice progressed over time but still showed no signs of tumor masses or enhanced proliferation28, macrophages were often localized inside the luminal epithelial layer of early lesions as demonstrated by co-staining of F4/80 and cytokeratin 8/18 (CK8/18) (Fig. 1f, Supplementary Table 1).