Sequence encoding the start codon and 127 downstream residues was replaced with a lacZ cassette inserted by homologous recombination. A floxed neo cassette that was included in the targeting vector was excised via in vivo cre mediated recombination, leaving a single loxP site immediately downstream of the tau-lacZ sequence. Normal transcript was undetected by RT-PCR analysis of homozygous mutant mice. (J:78008)