RNA voyeurism: A coming of age story.

Studies of gene expression are typically carried out by a molecular analysis that averages entire populations of cells in culture, or in a tissue. This approach cannot detect cell to cell variability, or collect subcellular information, such as spatial distribution. At the transcriptional level, it is evident that even a robust transcriptional response in ensemble measurements is not uniform among all cells in a population. At the post-transcriptional level, mRNAs and proteins can be trafficked to specific sub-cellular compartments allowing spatiotemporal regulation of gene expression, but these critical spatial relationships are lost with common molecular biology approaches. Through direct visualization of mRNA during the biogenesis process and analyzing the distribution of single mRNA molecules in cells we have gained a deeper understanding of gene expression at many levels. Recent technical advances have made these types of analysis more accessible than ever. The utility of this approach toward studying transcriptional events is underscored throughout many of the articles within this volume. Techniques such as fluorescent in situ hybridization (FISH) are being applied to single molecule studies in fixed cells with far-reaching results, but they are limited in their ability to provide information about the dynamic nature of mRNA in vivo, so methodology to visualize single mRNA molecules in living cells has become desirable. In this article, we will discuss the state-of-the-art tagging systems used for real-time imaging of mRNAs that have been developed. We will present an overview of how these approaches have been applied to impacting our view of gene expression.