Homologous recombination was used to replace the coding portion of exon 1 with GFPpA fused to a floxed neo cassette. Subsequent cre-mediated recombination removed the neo cassette, leaving GFPpA and one loxp site. The level of GFP fluorescence in developing CNS of heterozygous mice E9.5-E14.5 matched known expression of the endogenous protein.