Human transthyretin (TTR) has about 110 variants, more than 90 of which are associated with human amyloidosis. Several groups have generated transgenic mice that carry various mutant TTR genes. However, formation of mouse/human TTR heterotetramers has been shown to be inhibitory to dissociation and subsequent amyloid formation. To avoid the effect of mouse Ttr and produce humanized mice carrying different TTR variants at high efficiency, we first produced a null allele in the mouse transthyretin locus using targeting vector that contained a neomycin resistance gene flanked by lox71 and loxP. Then, through Cre-mediated recombination, we created a replacement allele that carried either a human normal (Val30) or mutant (Met30) TTR cDNA. This replacement resulted in a humanized TTR mouse with similar tissue-specific profile of human TTR as that of the endogenous mouse Ttr gene. The expression levels of human TTR mRNA and protein in the liver of homozygous human TTR (Val30/Val30) mice were about twice those of heterozygous mouse/human TTR (+/Val30) mice. However, the serum human TTR levels in the Val30/Val30 mice were much less than those in the +/Val30 mice. This contradictory expression was due to unstable Val30 tetramers caused by low binding affinity to mouse retinol binding protein.