In situ hybridization (ISH) has become a powerful and versatile method for the detection and localization of nucleic acid sequences within cells or tissue preparations. The technique provides a high degree of spatial information; specific DNA or RNA sequences can be located within a small subpopulation of cells, chromosomes or even individual cells within a tissue sample. ISH can be used to identify sites of gene expression, identify and localize viral infection, follow changes in specific mRNA synthesis, and map chromosomes.