We have always assumed that, since Down Syndrome occurs in people with an extra chromosome 21, the disorder should occur because there is too much of some protein coded for on that chromosome. A recent study suggests it may be due to little of a protein. It looks like what happens is a chunk of RNA gets over-expressed, which causes this protein* to be under-expressed.
In rats, they were able to increase the protein level by doing something to the RNA, but it couldn't fix damage that had already occurred. I predict in a few years we will have an in utero treatment after an early blood test for Down.
Of course, this doesn't mean that none of the characteristics of Down Syndrome are caused by over-production of some proteins, just that the under-production of one is involved. No doubt researchers will be busily looking at it all.