Huntington disease (HD) is caused by expansion of a CAG trinucleotide repeat in exon 1 of a novel gene. The HD protein (huntingtin) plays a critical role in early embryonic development since homozygous targeted disruption of the murine HD gene results in embryonic lethality by day 7.5. To rescue this phenotype by transgene based huntingtin expression it is therefore essential to express the protein early enough in development in the appropriate cells. Since YAC based transgenes are known to be regulated in an appropriate temporal and tissue-specific manner, we sought to rescue the embryonic lethality by breeding YAC transgenic mice expressing human huntingtin with mice heterozygous for the targeted disruption. We generated viable offspring homozygous for the disrupted murine HD gene but expressing human huntingtin derived from the YAC. This result clearly shows that YAC transgene based expression of huntingtin occurs prior to 7.5 days gestation. Additionally, we show that human huntingtin expression in YAC transgenic mice follows an identical tissue distribution and subcellular localisation pattern as that of the murine endogenous protein and that expression levels of 2-3 times endogenous can be achieved. This shows that human huntingtin under the influence of its native promoter, despite differences to the murine protein, is functional in a murine background and can compensate for loss of the murine protein. These results show that YAC transgenic approaches are a particularly promising route to producing an animal model for disorders associated with CAG expansion.