CD20 (MS4A1) is a transmembrane protein that is a member of the membrane-spanning 4A family. It plays a role in the humoral immune response by regulating B-cell proliferation and differentiation. Mutation of the CD20 gene results in impaired B-cell activation and T-cell–dependent humoral immunity. CD20 is expressed at the cell surface during B-cell ontogeny, from late pro–B-cells to memory cells. In normal tissues, it is found in bone marrow and immune system tissues. It is also found in acute and chronic B-cell lymphomas and leukemias, melanoma cancer stem cells, and subsets of Hodgkin lymphoma, myeloma, and thymoma. Staining is used to identify cells of the B-cell lineage and to distinguish B-cell neoplasms (positive) from T-cell neoplasms (negative). References: Proc Natl Acad Sci U S A. 1988;85(1):208-212. doi: 10.1073/pnas.85.1.208;Cancer Res. 2005;65(20):9328-9337. doi: 10.1158/0008-5472.CAN-05-1343;Manual of Diagnostic Antibodies for Immunohistology (2nd ed). London: Greenwich Medical Media; 2003;Janeway's Immunobiology (7th ed). New York: Garland Science; 2008;Textbook of Immunology (2nd ed). Amsterdam: Harwood Academic Publishers GmbH; 1996;J Immunol. 2013;191(6):3112-3118. doi: 10.4049/jimmunol.1202098.