Researchers have observed that, in the hippocampal tissue of APP/PS1 transgenic mice, in peripheral blood samples from LOAD patients, patients with early AD and advanced AD, the D-loop region was demethylated, the 12S rRNA, CYTB, and COX II-encoding genes were hypermethylated, and the mtDNA copy number was reduced. The gene discussed is APP; the disease is Alzheimer disease.