CD20 (cluster of differentiation 20) is a protein that is expressed on the surface of B cells, starting at the pre-B cell stage and also on mature B cells in the bone marrow and in the periphery. CD20 is not expressed on hematopoietic stem cells, pro-B cells, or plasma cells. This makes CD20 a specific marker for identifying B cells in various stages of their development, and it is widely used in the diagnosis and treatment of B-cell-related diseases, such as lymphomas and leukemias.