A tau-lacZ-loxP-tk-neo-loxP expression cassette replaced the coding exon and placed the tau-lacZ sequences under control of the endogenous regulatory sequences. Transient expression of Cre recombinase in C57BL/6 blastocysts excised the thymidine kinase and neomycin sequences. Expression of the tau-lacZ fusion protein in heterozygous mutant mice was detected in olfactory axon termini in individual glomeruli. (J:36595)