Annexin A1: One of the annexins, a family of proteins that share the property of binding calcium and phospholipids. The gene for annexin A1 (ANXA1) is upregulated in hairy cell leukemia. ANXA1 protein expression is specific to hairy cell leukemia. Detection of ANXA1 (by immunocytochemical means) reportedly provides a simple, highly sensitive and specific assay for the diagnosis of hairy cell leukemia.