The current method to create a chimeric murine embryo involves engrafting cells from a separate donor embryo. But, in December 10 online Nature Genetics, Pentao Liu and colleagues at The National Cancer Institute show that it is possible to engineer a mouse with cells that are genetically different from each other but are derived from the same embryonic stem cell.

Liu et al. used murine embryonic stem cells and found that mitotic recombination can be reproducibly induced using the Cre/loxP rare-cutter restriction enzymes. The mitotic recombination occurred at frequencies ranging from 4.2 ×10^-5(Snrpn) to 7 ×10^-3(D7Mit178) for single allelic loxP sites, and to 5 ×10^-2(D7Mit178) for multiple allelic lox sites, after transient Cre expression (Nat Genet 2001, DOI: 10.1038/ng788).

These results could offer a method to manipulate the expression of genes or collections of genes,...