OPRK1 and Alzheimer disease: The 34 studies showed that AD cases, compared to controls, have a higher degree of methylation of OPRK1, UQCRC1, AR, BDNF and HTERT in blood cells, BDNF, synaptophysin gene, CREB promoters, APOE, TREM 2, TBX2AR, SORBS3 and SPTBN4 in the brain, and lower methylation levels of 2-5a-synthetase gene in skin fibroblasts, PIN1, FAAH, ALOX5 and DR4 gene in blood cells, TNFA, COX-2, NF-kβ gene and S100A2 and CRTC1 in the brain tissue.