AIF1 (allograft inflammatory factor 1, IBA1) is an actin- and calcium-binding protein that enhances membrane ruffling and RAC activation. It is expressed in microglia in the central nervous system and in macrophages, where it plays a role in RAC signaling, phagocytosis, macrophage activation and function, vascular inflammation. Furthermore, it promotes the proliferation of vascular smooth muscle cells and T-lymphocytes. In immunohistochemistry, AIF1 serves as a marker for microglia and for macrophages. In cancer, it can also be used to differentiate malignant reticuloses (AIF1 positive) from astrocytomas (negative). In normal tissues, AIF1 has cytoplasmic and membranous positivity in immune cells, with particularly high expression in macrophages in the lung. References: J Neurochem. 2004 Feb;88(4):844-56, PMID: 14756805; Mol Med. 2015; 21(1): 233–241, PMID: 25569805; Neuroscience and Behavioral Physiology. March 2016, Volume 46, Issue 3, pp 284–290; J Toxicol Pathol. 2013 Mar; 26(1): 55–60, PMID: 23723569