ERBB2 and breast cancer: Genetically engineered mice that express the wild-type neu proto-oncogene under the control of the mouse mammary tumor virus promoter spontaneously develop focal mammary tumors after a prolonged latency period, suggesting that additional alterations are required for tumorigenesis.5 Interbreeding of mouse mammary tumor virus-neu mice with other genetically engineered mice models has identified numerous neu cooperating genes.6 However, this process is both costly and time consuming.