CD20 is a non-glycosylated phosphoprotein expressed on the surface of all mature B-cells. Membrane-spanning 4-domains, subfamily A, member 1, also known as MS4A1, is a human gene. This gene encodes a member of the membrane-spanning 4A gene family. Members of this nascent protein family are characterized by common structural features and similar intron/exon splice boundaries and display unique expression patterns among hematopoietic cells and nonlymphoid tissues. This gene encodes a B-lymphocyte surface molecule which plays a role in the development and differentiation of B-cells into plasma cells. This family member is localized to 11q12, among a cluster of family members. Alternative splicing of this gene results in two transcript variants which encode the same protein. The protein has no known natural ligand and its function is unclear. It is suspected that it acts as a calcium channel in the cell membrane. CD20 is expressed on all stages of B cell development except the first and last; it is present from pre-pre B cells through memory cells, but not on either pro-B cells or plasma cells. It is found on B-cell lymphomas, hairy cell leukemia, and B-cell chronic lymphocytic leukemia. It is also found on skin/melanoma cancer stem cells. CD20 is the target of the monoclonal antibodies rituximab, Ibritumomab tiuxetan, and tositumomab, which are all active agents in the treatment of all B cell lymphomas and leukemias.