The modified gene contains a single frt site downstream of exon 5 (left by FLP recombinase excision of a neomycin resistance cassette); a point mutation that results in replacement of one of two catalytically essential amino acids encoded by exon 6, exchanging aspartic acid at amino acid position 348 for glutamic acid (D348E); and an internal ribosomal entry site (IRES) followed by the E. coli beta-galactosidase gene (lacZ) with a nuclear localization signal downstream of exon 6. The enzyme encoded by this mutant allele is catalytically inactive. Mice with this transgene express the lacZ reporter protein under control of the endogenous promoter.