Transmembrane protein 119, Tmem119, is a bone formation related factor that belongs to a group of bone morphogenetic proteins (BMPs). It is a class IA single pass transmembrane protein¹.

Tmem119 promotes the differentiation of myoblasts (embryonic precursors of muscle cells) into osteoblasts and plays a role in numerous BMP signaling pathways^1,2.

Tmem119 is uniquely expressed in microglia cells and it is therefore used as a microglia-specific homeostatic marker that can assist in differentiating microglia cells from peripherally derived macrophages in the brain³. This marker is very important for diagnosing CNS disease: monocytes/macrophages are key cellular modulators of brain homeostasis. In the healthy brain resident microglia are the predominant macrophage cell population but when the blood-brain barrier is leaking peripheral monocytes/macrophages can infiltrate the brain and participate in CNS disease pathogenesis. Distinguishing these two monocytes/macrophages populations is often challenging and hence the existence of a highly expressed and robust microglia marker such as Tmem119 is very important⁴.