Notable among these are:Rubinstein-Taybi Syndrome (RTS), an autosomal dominant disease;89 Rett Syndrome (RS), an X-linkeddisease;90 fragile Xsyndrome, one of the most common forms of mental retardation;91 schizophrenia, in which evidenceindicates deficiencies in the extracellular matrix protein reelin,92 while finally and probably themost widespread form of dementia, Alzheimer's disease (AD). This evidence concerns the gene RELN and Alzheimer disease.