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. The gene discussed is ERBB2; the disease is neoplasm.