A unique TGF-β dependent molecular and functional signature in microglia was identified. Microglia are myeloid cells of the central nervous system (CNS) that participate in normal CNS function and disease. The study identified 239 genes and 8 microRNAs uniquely or highly expressed in microglia compared to other immune cells. These genes were enriched in microglia compared to astrocytes, oligodendrocytes, and neurons. The microglial signature was not observed in microglial lines or monocytes recruited to the CNS, but was observed in human microglia. TGF-β was found to be crucial for microglial biology, including the in vitro development of microglia expressing the microglial molecular signature and the absence of microglia in CNS TGF-β1 deficient mice. The study also identified a unique microglial microRNA signature, with 8 microRNAs highly expressed in microglia. Additionally, unique microglial surface molecules, such as P2ry12 and Fcrls, were identified. The study found that recruited monocytes do not acquire the microglial signature during neuroinflammation. Microglial cell lines do not express the adult microglial signature. TGF-β was found to be required for the development of microglia in vitro. In vivo, TGF-β played a critical role in microglia development, as TGF-β1 deficient mice showed a loss of microglial cells in the CNS. The study also identified that TGF-β is essential for microglial development and function. The findings provide insights into microglial biology and the possibility of targeting microglia for the treatment of CNS disease.A unique TGF-β dependent molecular and functional signature in microglia was identified. Microglia are myeloid cells of the central nervous system (CNS) that participate in normal CNS function and disease. The study identified 239 genes and 8 microRNAs uniquely or highly expressed in microglia compared to other immune cells. These genes were enriched in microglia compared to astrocytes, oligodendrocytes, and neurons. The microglial signature was not observed in microglial lines or monocytes recruited to the CNS, but was observed in human microglia. TGF-β was found to be crucial for microglial biology, including the in vitro development of microglia expressing the microglial molecular signature and the absence of microglia in CNS TGF-β1 deficient mice. The study also identified a unique microglial microRNA signature, with 8 microRNAs highly expressed in microglia. Additionally, unique microglial surface molecules, such as P2ry12 and Fcrls, were identified. The study found that recruited monocytes do not acquire the microglial signature during neuroinflammation. Microglial cell lines do not express the adult microglial signature. TGF-β was found to be required for the development of microglia in vitro. In vivo, TGF-β played a critical role in microglia development, as TGF-β1 deficient mice showed a loss of microglial cells in the CNS. The study also identified that TGF-β is essential for microglial development and function. The findings provide insights into microglial biology and the possibility of targeting microglia for the treatment of CNS disease.