An understanding of the histopathology of transgenic mouse mammary gland requires a comparison of glands from many different founder strains. The insertion and expression of a transgene, or the knock out of a gene may result in a abnormal growth and development of the gland which can only be appreciated by detailed examination of the entire growth, development, pregnancy, lactation and regression cycle. Mammary tumors from transgenic mice differ dramatically from "spontaneous" virus-induced tumors. Their histological and cytological phenotype is very dependent on the transgene expressed in the mammary gland. The lesions in the mouse show progressive changes in morphology and biological behavior that suggest neoplastic progression. These changes can be correlated with molecular lesions. The histology of the lesions found in the transgenic mammary gland closely resemble those found in the human breast. Given this remarkable resemblance, the biological and molecular changes found in the progressive mouse lesions should be reflected in the human breast. The rules of biological parsimony dictate that the transgenic mouse is an accurate model of human breast disease.
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