To this end we studied serial transplants of two F1 HER2/Delta16 mouse tumors in syngeneic immunocompetent hosts (mouse-derived isografts named MoMo1 and MoMo2) in comparison with two patient-derived xenografts (PDX named FO4 and TA18), established in our laboratory from human breast cancers expressing different levels of HER2, growing in immunodeficient NSG mice. The gene discussed is ERBB2; the disease is breast cancer.