Historically, autopsies have been a valuable resource for morphologic studies of the human breast, and have been used in conjunction with radiologic evaluation and epidemiology to provide information about population prevalence of pre-malignant and malignant disease. More recently, well-preserved post-mortem breast tissue has also been used to evaluate the status of genes and their expression. Using molecular techniques and immunohistochemistry, quantitation of gene expression and localization of proteins of hormone receptors, growth factor receptors, growth factors, cell-proliferation related antigens and proto-oncogenes can be evaluated in autopsy-derived breast tissue. Expression of the estrogen receptor (ER)3 gene at the mRNA and protein product levels has been evaluate d in breast tissue from infants, children, adolescent girls, and adult women in various phases of the menstrual cycle, pregnancy and post-menopausal. The results of these studies support a role for the ER gene in early as well as pubescent breast development, and also in normal cyclical and abnormal cell proliferation in the terminal-duct-lobular-units (TDLU) of adult women.