The ease of manipulating its genome has helped establish the mouse as the premier model organism for mammalian genetic studies. Both directed and random mutagenesis approaches, including the technologies of transgenesis and gene targeting in ES cells, have become commonplace. Genetically manipulated mice, often incorporating gene-based reporters, are frequently being used to model and understand mammalian development and disease processes. Fluorescent protein reporters currently represent a superior alternative to other gene-based reporters such as the bacterial lacZ or human placental alkaline phosphatase in that their visualization is non-invasive, and as such does not require chromogenic substrates. Fluorescence can be monitored in real-time in vivo and in situ, and has the added advantage in that it can be quantified. The prototype fluorescent protein reporter is green fluorescent protein (GFP, reviewed in [3]), which is derived from the bioluminescent jellyfish Aequorea Victoria [1].