Molecular Basis

The estrogen (ER) and progesterone (PR) nuclear hormone receptors are activated and appear to play a role in the development of more than half of all breast cancers.85-87 Positive ER and PR assays are associated with well-differentiated histology, negative lymph node status, diploid DNA content, low proliferation rate and a tendency for a relatively indolent clinical course.88,89 ER/PR-negative tumors are often associated with aggressive disease, including amplification of the HER2/NEU, MYC, and INT2

oncogenes, mutation of the TP53 gene, and increased expression of invasion- and metastasis-associated growth factors, growth factor receptors, and proteases.88,89

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