A new technique allows human genes to be expressed in mice in a way that much more closely resembles their normal expression patterns than was previously possible, paving the way for more accurate disease models.
Most mouse models are made using small transgenes that often exclude important cis-regulatory elements. Their expression is also influenced by the position of insertion into the genome. Wallace and colleagues developed a method that harnesses existing chromosome engineering technologies to allow entire human genes, including their complete regulatory regions, to replace the corresponding mouse loci. The authors tested their strategy by replacing the mouse 
-globin locus with the human sequence. This locus was chosen because its regulatory elements are extremely well characterized in both species.
The new method — which the authors named recombinase-mediated genomic replacement (RMGR) — relies on two different types of lox site, which are targets for the Cre recombinase. The two types are heterotypic — that is, a site of one type is not compatible with the other type. In both the genome of mouse embryonic stem (ES) cells and a BAC that carried the human locus, the authors used homologous recombination to insert a loxP site at the upstream end of the -globin regulatory region and a lox511 site at the downstream position.
When the human BAC was electroporated into mouse ES cells, Wallace and colleagues were able to identify cell lines that carried the desired construct on the basis of the absence or presence of selectable markers, which were introduced or removed during the genomic-replacement step. Although replacement lines were obtained at a low frequency, their production was both accurate and reproducible. Mice that were generated from the engineered stem cells expressed the human -globin genes in a pattern that closely resembled their expression in humans, proving the success of the strategy.
By carrying out an additional BAC engineering step before genomic replacement, the authors also generated a mouse line in which the human -globin locus contains a deletion of a key regulatory element. This line showed the features of human thalassaemia, illustrating the potential of RMGR to generate accurate models of human genetic disorders.