TREM2 and Alzheimer disease: The discovery that variants in TREM2 strongly increase the odds of developing not only AD, but also Parkinson’s disease, amyotrophic lateral sclerosis, and frontotemporal dementia underscores the important role that the innate immune system plays in neurodegenerative disease and suggests that TREM2 subserves a beneficial microglial response in a variety of pathologies [36,37].