COMT: catechol-O-methyltransferase; GMV: gray matter volume; PD: Parkinson's disease; Val: Valine; Met: Methionine; CSF: cerebrospinal fluid; AD: Alzheimer's disease; AA: adenine-adenine; AG: adenine-guanine; SNP: single nucleotide polymorphism. This evidence concerns the gene COMT and early-onset autosomal dominant Alzheimer disease.