CD20 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 non-lymphoid 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. It is expressed on both normal and malignant B cells. The expression of CD20 is observed in all mature B lymphocytes but lost in terminal differentiated plasma cells. CD20 is one of the most important markers for B cell. It labels almost all mature B-cell lymphoma, half of lymphoblastic lymphoma/leukemia and Reed-Sternberg cells in Hodgkin's disease. It has also been used to monitor down regulation of CD20 in patient treated with Rituximab for B-cell lymphoma.