While Trem2 haploinsufficiency intensifies tau pathology and brain atrophy in PS19 mice, complete Trem2 deficiency surprisingly protects against tau-mediated microglial activation and brain atrophy (Sayed et al., 2018), although some studies have reported no effect of Trem2 deficiency on tau pathology in PS19 mice (Leyns et al., 2017). Here, TREM2 is linked to Brain atrophy.