This is very clever - the X chromosome has a mechanism to shut itself down (which makes sense; otherwise cells in women would have twice as many gene products from the X chromosome as cells from men).
The linked research report[1] uses that mechanism, Xist, to shutdown chromosome 21, the extra chromosome whose presence causes Down syndrome. In its present form, it would need to be optimized for each potential patient and is unlikely to be used as a treatment paradigm, but the biological approach is clever.