Scientists have been able to introduce most of a human chromosome into mice - and make the most successful recreation of Down's syndrome so far.
Mice have been created which show signs of Down's syndrome
But what will this enable researchers to do?
What was being looking at?
The researchers were aiming to transfer as much of a human chromosome into a mouse embryonic stem cell as possible.
Humans have 23 pairs of chromosomes which contain all human genes.
Males and females share 22 of these chromosome pairs; the 23rd is the sex chromosome, where women have two X chromosomes and men have one X and one Y.
Down's syndrome belongs to a class of disorders known as aneuploidies in which individuals have the wrong number of chromosomes.
Aneuploidies occur in at least 5% of all pregnancies and are a significant cause of illness, death and miscarriage.
People with Down's are born with three copies of chromosome 21.
There are an estimated 60,000 people in the UK who have Down's syndrome. They can expect to live between 40 and 60 years.
What has been achieved?
The researchers, led by Victor Tybulewicz at the National Institute for Medical Research and Professor Elizabeth Fisher from the Institute if Neurology at University College London, were able to add about 90% of human chromosome 21 - which contains around 250 genes - into the embryonic stem cells of mice.
They did this by extracting all chromosomes from a human cell. These were then squirted onto mouse embryonic stem cells, each of which will absorb one chromosome at random.
Stem cells that took up chromosome 21 were then isolated and injected into mouse embryos.
These were then replaced in the mother, whose offspring were shown to have the extra copy of chromosome 21, which also had problems with memory, in brain function and in the formation of the heart, similar to those that can occur in people with Down syndrome.
Why is this significant?
The scientists involved in this work have been trying to get to this stage for 13 years.
Until now, it had only been possible to transfer single genes or, at best, fragments of human chromosomes into mouse cells, meaning study of the effects of disease were limited.
This achievement of transferring almost the whole of the chromosome into the embryonic stem cells will give a much fuller picture.
How will it help in the study of Down's syndrome?
It is hoped it will enable scientists to examine the role individual genes play in Down's syndrome, and why those with the condition are particularly susceptible to diseases like leukaemias and autoimmune disorders.
Understanding why this is should help scientists work out what can be done to help people with DS, and may also shed light on how the diseases work in people who do not have the condition.
It means scientists will be able the study the condition, without having to consider creating human embryos with the condition.
Researchers say it will also help in the study of other aneuploidies, such as Edward's syndrome, where there are three copies of chromosome 18 and Patau's syndrome, where an extra copy of chromosome 13 is present. Both prove fatal in childhood.
And they hope it will help unravel the picture of individual genes responsible for complex conditions, such as diabetes, and also help create artificial chromosomes for gene therapy.
Are there any concerns over the development?
The pressure group Human Genetics Alert has raised concerns over the creation of virtually whole chromosomes from another species.
It has suggested scientists may be creating a creature somewhere between a mouse and a person, particularly if work progresses so more than one chromosome can be added.