The biology of mammary transgenes: five rules.

The development of hyperplasias, dysplasias, and mammary tumors has been studied extensively in transgenic mice. It is now becoming clear that transgenes activate and participate in oncogenic pathways that govern the events surrounding neoplastic progression in transgenic mice. The oncogenic pathways control mammary growth, development, and neoplastic progression. Some of the key features of transgenic biology can be expressed in the following rules: (1) Mammary development is related to the type and amount of transgene expressed; (2) dysplasias and tumors develop from secondary mutations; (3) the transgenes determine tumor phenotype; (4) transgenes may activate dominant oncogenic pathways; and (5) the oncogenic pathway determines prognosis. The study of the comparative pathology of mammary tumorigenesis in many strains of transgenic mice provides examples of these principles.