TREM2 and Alzheimer disease: Heterozygous mutations in TREM2, such as the R47Hhet mutation, are associated with Alzheimer’s disease (Guerreiro et al., 2013; Jonsson et al., 2013), whilst homozygous missense TREM2 mutations such as T66M cause Nasu-Hakola disease, a rare early-onset dementia with bone cysts (Paloneva et al., 2002; Guerreiro et al., 2013).