Annexin A1, also known as Lipocortin I, belongs to a family of Ca(2+)-dependent phospholipid binding proteins that are preferentially located on the cytosolic face of the plasma membrane. It promotes membrane fusion and is also involved in exocytosis. Since Annexin A1 has phospholipase A2 inhibitory activity to bind from two to four calcium ions with high affinity, this protein may have potential anti-inflammatory activity. The detection of this protein by immunocytochemical means reportedly provides a simple, highly sensitive and specific assay for diagnosis of hairy cell leukemia.