Cluster of differentiation 22 (CD22) is a glycoprotein that appears on the cell membrane of B-cells and the cytoplasm of late pro- and early pre-B lymphocytes. Anti-CD22 is a useful pan-B reagent that allows for the identification of neoplastic and normal B lymphocytes in the peripheral blood. CD22 is overexpressed in hairy cell leukemia. Expression is only present in the late stages of B-lymphocyte differentiation, and therefore may support the identification of mature B-cell leukemia. Some studies have suggested that CD22 may be involved in metastasis of lung cancer cells.