Alternatives to syngeneic mouse cell lines are tumour-prone genetically modified mice (Fantozzi and Christofori, 2006), such as those expressing a polyoma middle T (PyMT) (Guy et al., 1992a) or ErbB2/neu (Guy et al., 1992b) transgene in the mammary gland. This evidence concerns the gene ERBB2 and neoplasm.