The majority of exon 3 was replaced with a tau-lacZ reporter and a floxed PGK-neo cassette. A point mutation was introduced into the initiator ATG to create a stop codon. X-gal staining showed beta-galactosidase activity was present in mutant brains. Endogenous mRNA was absent as shown by Northern blot, and a two-site immunoassay confirmed that no peptides were detected in mutant plasma. (J:89244)