A loxP-flanked neomycin selection cassette replaced a genomic fragment containing exon 10. The neomycin cassette was removed via Cre-mediated recombination. This allele mis-expresses the 3R-tau isoform after P18 in heterozygous and homozygous mice, when it is not seen in wild-type mice. 4R-tau isoform expression is unaffected. (J:221311)