To analyze the effect of NRTI on spontaneous tumor development and acquisition of resistance to treatment, we used MMTV-HER2/Neu mice carrying in their germline a transgenic cassette comprised of rat HER2/Neu cDNA under the control of the promoter from the long terminal repeat of mouse mammary tumor virus (MMTV) that drives tissue-specific expression of the proto-oncogene in the breast epithelium (23). Here, ERBB2 is linked to neoplasm.