ERBB2 and neoplasm: Consistent with tumor growth and metastasis data above, we found the CSC content is significantly higher in lapatinib-treated MMTV-HER2/MMTV-MED1 tumors and is close to that of the vehicle-treated MMTV-HER2 control, which was further confirmed by mammosphere formation assays (Figures S3G–S3I).