TREM2 and Alzheimer disease: Mutations and loss-of-function for TREM2 have been associated with deficiencies in phagocytosis, the innate-immune system, axonal and synaptic abnormalities, amyloidogenesis and progressive dementia in progressive neurological diseases of the human CNS including polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL; also known as Nasu-Hakola disease [6–11] as well as more recently in sporadic amyotrophic lateral sclerosis (ALS) [11] and in Alzheimer’s disease (AD) [6–15].