In a later study, Xu et al. examined methylation in mitochondrial cytochrome b (CYTB) and COX II in the hippocampi of APP/PS1 AD mice in comparison to C57BL/6J mice and found hypermethylation and decreased mitochondrial DNA copy numbers in the hippocampus of these specific genes in the AD transgenic mice [122]. This evidence concerns the gene PSEN1 and Alzheimer disease.