CD45R/B220 is an isoform of the CD45 protein, a member of the protein tyrosine phosphatase (PTP) family, and serves as a critical regulator of T- and B-cell antigen receptor signaling. It is predominantly expressed on the surface of B cells, from early pro-B cells to mature and activated B cells, making it a commonly used pan-B cell marker in mice. Additionally, CD45R/B220 is also found on certain subsets of activated T cells, lymphokine-activated killer cells (LAK), NK cell progenitors, and T cells in mutant mice. This molecule plays a crucial role in lymphocyte development and antigen signaling, and its expression is regulated during different stages of B cell maturation.