The color of mice: in the light of GFP-variant reporters

The mouse currently represents the premier model organism for mammalian gen etic studies. Over the past decade the production of targeted and transgeni c lines of mice has become commonplace, with current technology allowing th e creation of mutations at base pair resolution. Such genome modifications are becoming increasingly elaborate and often incorporate gene-based report ers for tagging different cellular populations. Until recently, lacZ, the b acterial beta-galactosidase gene has been the marker of choice for most stu dies in the mouse. However, over the past 3 years another valuable reporter has emerged, and its attractiveness is reflected by an explosion in its us e in mice. Green fluorescent protein (GFP), a novel autofluorescent genetic reporter derived from the bioluminescent jellyfish Acquorea victoria, curr ently represents a unique alternative to other gene-based reporters in that its visualization is non-invasive and so can be monitored ill real-time in vitro or in vivo. It has the added advantage that it can be quantified by, Tor example, flow cytometry, confocal microscopy, and fluorometric assays. Several mutants of the original wild-type GFP gene that improve thermostab ility and fluorescence have been engineered. Enhanced GFP is one such varia nt, which has gained popularity for use in transgenic or targeted mice. Mor eover, various GFP spectral variants have also been developed, and two of t hese novel color variants, enhanced yellow fluorescent protein (EYFP) and e nhanced cyan fluorescent protein (ECFP), can also be used in mice. Since th e spectral profiles of the ECFP and EYFP color variants are distinct and no n-overlapping, these two reporters can be co-visualized, and are therefore ideal for in vivo double-labeling or fluorescent energy transfer analyses. The use of GFP and its color variants as reporters provides an unprecedente d level of sophistication and represents the next step in mouse genome engi neering technology by opening up the possibility of combinatorial non-invas ive reporter usage within a single animal.