The most famous genetic decease is Daun's syndrome. It is caused by tripsony of 21th chromosome. If you are a healthy person, you have two copies of this chromosome, and if you have three copies of the chromosome, you have a trisomy and you are ill. So all the problem is in extra chromosome.
Team of biologies from University of Massachusetts Medical School in Worcester discovered a new way to treat Daun's syndrome. The key for the treatment is the fact that all women have... extra chromosome too! As you know, all women care XX genotpe, and all men care XY genotpe. Thus, as soon as all men live successfuly with only 1 chromosome, women have one extra chromosome. However, women are not ill because there is a gene, called XIST, which inactivates one X chromosome by covering it by special RNA.
What scientists do is to put XIST into one of 21th chromosome and made it inactive. The experiment was conducted on iPSC. This method will allow to treat other chromosomic desease in future.
Jiang, J. et al. Nature http://dx.doi.org/10.1038/nature12394 (2013).