A cassette containing a tau-lacZpA reporter with a floxed tk-neo component was inserted in the place of exons 1-6 by homologous recombination, and remained under transcriptional control of the endogenous promotor. The exons encoded the first 185 amino acids, which contain domains crucial for DNA binding and transactivation of the protein binding site. Subsequent cre-mediated recombination removed the tk-neo cassette. In situ hybridization indicated lack of endogenous gene expression in homozygous mutants. The expression pattern of lacZ recapitulated that of the endogenous gene. (J:88585)