Compositions and uses of mGluR.sub.5 antagonists for the treatment and
inhibition of amyloid precursor protein (APP), A.beta. protein, and APP
proteolytic products in Alzheimer's disease, Fragile X Syndrome, autism,
and Down's Syndrome are provided. The invention provides methods for
diagnosing Fragile X Syndrome via the assessment of A.beta..sub.1-42
levels in blood plasma.