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.