Microarrays are small chips a few square inches in size on which spots of DNA have been imbedded. A typical chip may contain thousands of spots, each spot composed of multiple copies of a small sequence of DNA. In the living cell nucleus, sections of DNA are copied (“transcribed”) into a dual complementary molecule, RNA, which is the scaffolding for the synthesis, outside the cell nucleus, of cellular proteins. RNA can be extracted from tissue, and tiny luminescent beads can be chemically attached to RNA molecules obtained from tissue cells (e.g., from breast cancer cells). Each RNA molecule contains a sequence of bases that binds to a specific DNA sequence. When a suspension consisting of many RNA molecules from a tissue sample is applied to a microarray, the RNA molecules bind to the complementary DNA sites. By measuring the luminosity of each DNA spot, the relative concentration of each kind of RNA in the tissue sample can be estimated. From these concentrations, one can infer relative activity of genes—how much RNA is produced by various parts of the cell DNA in the tissues sampled.