AIF1, also named as G1, IBA1 or daintain/AIF-1, is a 143 amino acid cytoplasmic, inflammation response scaffold protein. It is constitutively expressed in monocytes and macrophages and is known to be involved in macrophage activation. It is a marker of activated macrophage. Despite a lack of detailed knowledge on the in vivo physiological functions of AIF-1, there is growing evidence that shows its aberrant expression contributes to the pathogenesis of many autoimmune diseases, including rheumatoid arthritis. (20944424). AIF1 is an actin-binding protein that enhances membrane ruffling and RAC activation. It enhances lymphocyte migration and the actin-bundling activity of LCP1. AIF1 plays a role in RAC signaling and in phagocytosis and a role in vascular inflammation. AIF1 promotes the proliferation of vascular smooth muscle cells and of T-lymphocytes. It is also as a microglial cell marker.