ES cells containing the Ttrtm1Kymm null allele that serves as acceptor in recombination mediated cassette exchange were electroporated with a replacement vector and a cre recombinase expression vector. The replacement vector consisted of a wild-type human TTR cDNA and a FRT-flanked puromycin resistance gene flanked by a lox66 and a loxP site. Northern blot analysis indicated gene expression in the brain, liver and eye of homozygote mice was similar to endogenous expression in wild-type mice. Human but not mouse TTR protein was detected in the liver and serum of homozygote mice in immunoblot assays.