The exon containing the entire coding region of the gene was replaced with a GFP-PGK-neo cassette via homologous recombination. In situ hybridization analysis of E10.5 embryos demonstrated the absence of gene expression in homozygous mutants. GFP transcripts were detected in mutant animals in a pattern indistinguishable from the endogenous mRNA but no fluorescent signal could be detected. (J:46668)