The mouse mammary tumor virus (MMTV) promoter-driven polyomavirus middle T antigen (PyVT) and the MMTV-driven erythroblastic leukemia viral oncogene homolog 2 (Erbb2) mouse models of breast cancer are ideal for oncogenesis studies due to colony stability, predictable tumor growth behavior, and similarity to human neoplasms. This evidence concerns the gene ERBB2 and breast cancer.