Introduction
The study of gene expression in breast tumors, as in most solid tumors, is hampered by heterogeneity of the cancer cells as well as contamination by other cell types present in the tissue. cDNA microarrays are a novel tool for the investigation of gene expression, and consist of thousands of different cDNA clones spotted onto known locations on glass microscope slides. These slides/microarrays are then hybridized with differentially labeled cDNA populations made from the mRNAs of two different samples. The primary data obtained are ratios of fluorescence intensity representing the ratio of concentrations of mRNA molecules that hybridized to each of the cDNAs represented on the array.