TREM2 and Parkinson disease: Recently, mutations in the innate immune receptor-adaptor duo, triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12), have been reported in a spectrum of neurodegenerative disorders including Nasu-Hakola disease (NHD; or polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, PLOSL) [8–11], frontotemporal dementia (FTD) [12–17], Alzheimer’s disease (AD) [13, 17–23], Parkinson’s disease (PD) [14], amyotrophic lateral sclerosis (ALS) [24], and essential tremor [25].