Iba-1, designated as allograft inflammatory factor 1 (AIF1), is a 17 kDa cytoplasmic calcium-binding protein that is constitutively expressed in monocytes, macrophages and CD3+ lymphocytes, but can also be induced by cytokines and interferon α. Iba-1 is a key molecule that co-localizes with Rac in the membrane ruffling process, a feature of actively migrating cells. Iba-1 may promote the proliferation of vascular smooth muscle cells and T-lymphocytes, enhancing lymphocytic migration, and plays a role in vascular inflammation. In addition to leukocytes, Iba-1 expression has been described in the testis and spleen, and weakly expressed in the brain, lung and kidney. Within the brain, Iba-1 was specifically localized in microglia. Iba-1 may be an indicator of macrophage activation in the body since it is also expressed in various human immune-related tissues, including rheumatoid arthritis, inflammatory skin disorders, system sclerosis, and cardiac allograft vasculopathy. A recent study suggested that Iba-1 expression was associated with breast ductal tumors and may be helpful for the prognosis and therapy of breast cancer.