Scientists succeeded in removing the extra copy of chromosome 21 in cell cultures derived from a person with Down syndrome. Persons with this condition have three copies of this chromosome. The method would not lead to a treatment for Down syndrome, but could lead to treatments for some conditions associated with it, such as leukemia. Comparing tissue development from cell cultures from the same individual, with and without the extra chromosome, could provide clues to developmental delays and early aging problems in Down syndrome.