DRD4 and early-onset autosomal dominant Alzheimer disease: More recent studies add dopaminergic system genes (i.e., catecholamine-O-methyltransferase (COMT); dopamine receptor D4 (DRD4), and dopamine transporter 1 (DAT1)), Apolipoprotein E (APOE, ε4 allele; commonly known in the literature for its role in Alzheimer’s disease), peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α; important for expression of clock genes [57]), and the arylhydrocarbon receptor (AHR) to the list of candidate genes studied.