How much human DNA can mice hold?

For many years, scientists have created transgenic mice by adding individual human genes to the mouse genome. A research group in Japan has unexpectedly found that it can also introduce large fragments of human chromosomes, some probably containing more than 1,000 genes.

When single genes are added to the mouse genome, they fit into mouse chromosomes at random locations, which may alter the timing, location, and intensity of the human gene's activity. In contrast, the chromosome fragments remain independent, they are copied and distributed normally when cells divide, and their genes appear to behave as they do in people, report Kazuma Tomizuka of Kirin Brewery Co. in Kanagawa, Japan, and his colleagues in the June Nature Genetics.

Moreover, in several cases, a human chromosome fragment made it into the germ cells of a mouse, enabling the mouse to pass the fragment to its offspring.