Immunohistochemical analyses using tissue microarrays showed that gingipain immunoreactivity in AD brains and that gingipain immunoreactivity significantly correlates with tau and ubiquitin loads and AD diagnosis. Using quantitative Polymerase Chain Reaction, the authors identified P. gingivalis DNA in the AD brains which were lysine gingipain-positive. This evidence concerns the gene MAPT and Alzheimer disease.