Microglial subtypes, such as proliferative-region-associated microglia (PAM), axonal tract-associated microglia (ATM), triggering receptor expressed on myeloid cells 2 (TREM2)-microglia, TREM2-independent microglia, and neurodegenerative disease-associated microglia (DAM) have been discovered recently (Wang et al., 2015; Keren-Shaul et al., 2017; Sousa et al., 2018; Li et al., 2019; Stratoulias et al., 2019). This evidence concerns the gene TREM2 and neurodegenerative disease.