TREM2 and Alzheimer disease: Mutations and single nucleotide polymorphisms (SNPs) in the TREM2–DAP12 axis have been related to a variety of neurodegenerative disorders such as Nasu–Hakola disease, frontotemporal dementia, AD, PD, amyotrophic lateral sclerosis (ALS), and essential tremor (for reviews see [13,44,45]).