Furthermore, colonization of germ-free mutant APP/presenilin 1 (APP/PS1) transgenic mice with GM derived from conventionally-raised APP/PS1 transgenic mice drastically increased the cerebral Aβ pathology compared to wild type (WT) and germ-free transgenic mice,25 signifying that GM may play a causative or contributory role in AD onset and progression. The gene discussed is PSEN1; the disease is Alzheimer disease.