TMEM119 (transmembrane protein 119) is a transmembrane protein with dual roles in bone metabolism and neuroinflammation. In bone biology, TMEM119 promotes myoblast differentiation into osteoblasts and enhances bone mineralization, likely through BMP2 production and the BMP-RUNX2 pathway, with upregulation of the transcription factor ATF4 1. TMEM119 upregulation contributes to osteoblast bone formation and may counteract oxidative stress-induced osteoporosis 1. In the central nervous system, TMEM119 functions as a microglial homeostatic marker. Microglial TMEM119 binds amyloid-β oligomers and promotes their clearance through low-density lipoprotein receptor 1-mediated degradation 2. In Alzheimer's disease, TMEM119 expression decreases in microglia, and TMEM119 deficiency accelerates disease progression in mouse models 2. Pharmacological enhancement of TMEM119 expression with small molecules improved amyloid clearance and cognitive function even at mid-disease stages 2. However, TMEM119 is not exclusively microglial; it is also expressed by follicular dendritic cells and brown adipose tissue, and expression significantly decreases under cellular stress conditions 3. In Alzheimer's disease brains, TMEM119+-microglia prevalence substantially decreased compared to controls 4. These findings position TMEM119 as a promising therapeutic target for Alzheimer's disease, though interpretation as a pan-microglial marker requires caution.