With more than 20,000 genes in the human genome now identified and a similar number of genes in the rat and mouse genome known (Gibbs et al., 2004), elucidating their function has become a major challenge. An important contribution to understanding genetic pathways was made by the development of reporter genes. A reporter gene is a gene whose product can be readily detected and either fused to the gene of interest or cloned instead of that particular gene. Optical reporter genes are the most commonly used and widely developed. Throughout the years, multiple genes have been cloned from a variety of organisms that emit light via bioluminescence or fluorescence, in multiple distinguishable wavelengths. An emerging new class of reporter genes encode for proteins with an affinity for radioisotopes or positron emitter probes. These receptors, transporters, or enzymes can provide quantitative images upon administration of suitable radiolabeled probes (Serganova et al., 2007). Magnetic resonance imaging (MRI) reporter genes are unique among all the reporter genes used with the various imaging modalities, since they can provide information about gene expression that can be co-registered with soft tissue anatomical and functional information (Gilad et al., 2007b). A key feature of reporters for MRI (and other noninvasive imaging techniques) is that they enable serial temporal imaging within the same subject. This is particularly useful for studying dynamic processes, for example, the migration of stem cells and progenitors, neuronal plasticity, the mechanisms of development and adaptation, disease progression, and response to trauma or illness, all of which require serial imaging of the same individual over time. The process of uncovering reporter genes for molecular-genetic imaging usually begins by searching for a gene with desirable imaging properties, such as the green fluorescent protein (GFP)(Shimomura et al., 1962). Advances in recombinant DNA technology have made it possible to clone and express these reporters in both prokaryotic and eukaryotic cells (de Wet et al., 1985; Chalfie et al., 1994). For most candidate reporter genes, a significant improvement in detection efficiency has been achieved after mutations were systematically introduced (Cormack et al., 1996), or by mutations that created variants that could be detected with multiple excitation frequencies (Shaner et al., 2004; Zhao et al., 2005). Directed evolution is a process of generating a library of genes with random mutations and screening for mutants with a superior signal (eg, enhanced signal), and then repeating the same process using the improved protein as a starting point for mutagenesis until a significant improvement in detection efficiency is achieved. For example, this process was recently applied to the bacterial cytochrome P450-BM3, a putative MRI sensor for dopamine, and improved its sensitivity 100-fold (Shapiro et al., 2010). Over the past decade, viral vectors have been widely used for gene delivery to the central nervous system (CNS). Viruses of different types (eg, adeno-associated virus, herpes simplex virus, and lentivirus) have several advantages: they can carry a large amount of genetic material, from 8 to 150Kb; they can infect a wide range of cells, including non-dividing cells (as most of the cells in the CNS are); and they lead to transient or stable gene expression, depending on the virus type (Davidson and Breakefield, 2003). Lentiviruses are derived from the immunodeficiency virus type 1 (HIV-1). These are retroviruses in which the genetic material is stored as RNA, and the viral particles are coated with a lipid envelope with embedded …