ERBB2 and neoplasm: Given that tamoxifen resistance modeled in NSG-Pro mice mimics clinical resistance in patients, the unraveling of an early ERBB2 escape mechanism in PDX tumors might be used for precision-guided therapy of recurrent disease in the patient that cannot be readily predicted simply on the basis of ERBB2 protein levels or ERBB2 amplification status in the patient’s primary tumor.