The molecular cytology of gene expression: fluorescent RNA as both a stain and tracer in vivo.

For more than 60 years, RNA has been detectable in fixed cells and tissues by relatively specific staining methods. More recently, it has become possible to study RNA in unfixed, live cells. This review article describes how the intracellular dynamics and localization of RNA in vivo can be studied by microinjection of fluorescent RNA into cells- an approach we have termed Fluorescent RNA Cytochemistry. Depending on the particular RNA species under investigation, Fluorescent RNA Cytochemistry can operate as a "stain" to reveal intracellular sites at which a given RNA resides, or as a "tracer" to allow movements of a dynamically translocating RNA to be followed in the living cell. Several examples of Fluorescent RNA Cytochemistry are presented, collectively illustrating the range of applicability this approach offers in the toolbox of gene expression, studied as in vivo cell biology.