ERBB2 and cancer: To rule out that HER2+ cells might be macrophages that engulfed early cancer cells in the mammary tissue and migrated to the lung, we tested the fraction of cells in the lungs in MMTV-HER2 animals that might be double positive for HER2 and macrophage markers (F4/80) (Supplementary Fig. 3A, Supplementary Table 7).