The structure of microarray experiments. (a) To obtain gene-expression profile data from a cDNA microarray, or chip, RNA is first extracted from an infected cell. The RNA is then reverse-transcribed and labeled ('sample preparation') and prepared RNA is hybridized to the chip. (b) Protein microarrays may have either antibodies or antigens arrayed as probes. Antibody probes can be used to detect antigens from an infected cell, and vice versa, following sample preparation and labeling. In both cases (a,b) the hybridized chip is scanned and the image processed to provide corresponding profiles.