Glowing fish have been modified to carry a gene from jellyfish.  This gene produces a protein – called Green Fluorescent Protein, or GFP – that glows green when blue light is shone on it.  Animals that have been modified using genes from other organisms are called transgenic.

We inject the jellyfish gene into fertilised fish eggs.  These grow into adults carrying the jellyfish gene.  We breed these fish to produce the glowing embryos used in our research.

We use bits of the fish’s own DNA to control where the GFP is made.  If we want to label the muscle cells for instance, we need to find a piece of zebrafish DNA that normally tells a protein to be made in muscle. This bit of DNA – called a promoter – is like a switch, in each cell it tells a gene to be on or off.  
Joining the promoter sequence to the gene for GFP makes what is called a transgene.  Such a transgene has all the information needed to direct the production of GFP in a specific cell type. 

We can ‘light up’ different cells and organs in the embryo.  This means we can follow these cells as the embryo develops. 

By tracking fluorescent cells in mutant embryos we can study how the defect affects normal cell functions.

Understanding the promoter sequences used to make transgenic fish is also important.  Knowing how genes are switched on and off is critical to understanding disease and development. 

A very interesting and useful finding is that promoters from one species are often the same in other species, like genes, they are conserved.  To demonstrate this conservation it is possible to link the promoter from one animal to GFP and put it in another species.  A frequent outcome of such an experiment is that the promoter makes the pattern of GFP the same in each species.