Conventionally Downs syndrome has been deemed irremediable. However recently some researchers have aimed to develop ‘chromosome therapy’ for Downs Syndrome.
This has been done by:
1) inserting a TK-NEO marker that can be selected against in order to eliminate one copy of chromosome 21
2) inactivating one of the copies of chromosome 21 by inserting into it a copy of the XIST gene that normally inactivates one of the two X chromosomes in female cells.
Even though these procedures have provided proof of principle, they have not been applied to due to their requirement of elaborate and cumbersome multi-step genetic engineering.