Non-human animal cells and non-human animals comprising a humanized TTR locus comprising a beta-slip mutation and methods of using such non-human animal cells and non-human animals are provided. Non-human animal cells or non-human animals comprising a humanized TTR locus comprising a beta-slip mutation express a human transthyretin protein or a chimeric transthyretin protein, fragments of which are from human transthyretin. Methods are provided for using such non-human animals comprising a humanized TTR locus to assess in vivo efficacy of human-TTR-targeting reagents such as nuclease agents designed to target human TTR.