Nonisotopic in situ hybridization can be used to determine the cellular location and relative levels of expression for specific transcripts within cells and tissues. RNA in specimen preparations is hybridized with a biotin- or digoxigenin-labeled probe, which is generally detected by fluorescence or enzymatic methods. Fluorescence in situ hybridization (FISH), probably the most widely used method, is described here, along with amplification of weak FISH signals. Nonisotopic probes can also be detected by enzymatic reactions using horseradish peroxidase or alkaline phosphatase, as described here. (c) 2007 by John Wiley & Sons, Inc.