Imaging gene expression in live cells and tissues.

Monitoring gene expression is crucial for studying the responses of gene therapy and clarifying gene function in various environments. Molecular imaging is a powerful tool for noninvasive visualization of gene expression. This article summarizes the current status of fluorescence and bioluminescence imaging (BLI) of gene expression in live cells and tissues, with the emphasis mainly on the early studies that pioneered the field. First, we describe fluorescence imaging of gene expression with a wide variety of fluorescent proteins. Next, we discuss the strategies for BLI of gene expression. Besides incorporating the reporter gene into the host DNA, mRNA-based BLI of gene expression is also briefly mentioned. Last, the construction of doubleand triple-fusion reporter genes is presented. Because no single imaging modality is perfect and sufficient to obtain all of the necessary information for a given question, combinations of multiple molecular imaging modalities can offer synergistic advantages over any modality alone. Noninvasive optical imaging of gene expression has revolutionized biomedical research, and the progress made over the last decade should allow molecular imaging to play a major role in the field of gene therapy. For basic and preclinical research, optical imaging is indispensable for imaging gene expression. However, for clinical imaging of gene expression, positron emission tomography (PET) holds the greatest promise.