For example, the tryptophan hydroxylase-1 (TPH1) A218C allele, monoamine oxidase A (MAOA) variable number tandem repeat (VNTR), and BDNF Val66Met allele were associated with depression in females with AD, with significantly increased likelihood of comorbid AD and depression in homozygous TPH1 A-allele and MAOA VNTR carriers [48]. Here, TPH1 is linked to Alzheimer disease.