TREM2 and Alzheimer disease: Homozygous or compound heterozygous mutations in TREM2 are known to cause Nasu-Hakola disease (NHD) or an early-onset frontotemporal dementia (FTD)-like syndrome, while rare variation in TREM2 increases risk for AD, and may also increase risk for FTD, Parkinson’s disease, and amyotrophic lateral sclerosis [8–10].