Scientists have transplanted human chromosome 21 in mice in a breakthrough that could yield key insights on disorders such as Down's syndrome. Chromosome 21 is the smallest of the 23 pairs of human chromosomes. Children suffering from Down's syndrome inherit three copies of chromosome 21, instead of two. The achievement caps 13 years of research at London's National Institute for Medical Health and the Institute of Neurology. In their report in the journal Science, researchers describe how they mixed the human chromosome with mouse embryonic stem cells.