CD45R/B220 is an isoform of the CD45 protein, specifically expressed on murine B-lineage cells, including pro-B, pre-B, and mature B cells, as well as a small subset of T cells and NK cells. It is a 220 kDa surface molecule and a member of the protein tyrosine phosphatase (PTP) family, which plays a crucial role in regulating lymphocyte development and antigen receptor signaling. B220 is commonly used as a pan B-cell marker in mice due to its restricted expression on B cells, making it a valuable tool in immunological research and flow cytometry.