The complementary DNA (cDNA) of a derivative of green fluorescent protein (GFP) from a jelly fish, was introduced  to the genome encoding G-subtrate by knock-in technique.  G-substrate is specifically localized in cerebellar Purkinje cells  (diameter of 30 m in mice), which are the largest neurons in the mammalian central nervous system.  Cerebellum plays important roles in motor learning, motor coordination and non-declarative memories.  Cerebellar Purkinje cells is thought to be a essential neuronal component for these brain functions.  Neuronal plasticity (flexibility of neuronal  transmission) called, long-term depression, underlies these cerebellar functions.
    By expressing GFP, the shape and tracks of Purkinje cells are easily visible under fluorescence microscope.  In the picture, the Purkinje cell bodies have strong green fluorescence and a round shape.  Purkinje cell dendrites, which enter the molecular layer, are observed as spurs going upward.  Also bundles of axons going to cerebellar nuclears are also observed.
    The GFP introduced into the specific neurons allows us not only to track the neuronal pathways but also the shape of cells.  The GFP gene is often used as a reporter of the expression under a certain promoter or the general expression of the target protein. 

Many fluorescent bacteria, fly, yeast, and mammalian cells are developed using GFP.  In addition to fluorescent mice, "fluorescent pigs" were generated by Taiwan's group by transgenic technology.  If you want to see glow-in-dark FLUORESCENT PIG, Click here.