The A20 monoclonal antibody reacts with mouse alloantigen CD45.1 (Ly5.1) expressed by all hematopoietic cells except mature erythrocytes and platelets in mouse strains RIII, SJL/J, STS/A, and DA. Clone A20 does not react with leukocytes that express CD45.2 alloantigen. CD45.1 is an allelic form of CD45 (leukocyte common antigen), a 180 - 240 kDa type I transmembrane glycoprotein. CD45 is a member of the protein tyrosine phosphatase (PTP) family and functions in a number of immunoregulatory processes, including cell activation, growth, lymphocyte development, differentiation, and oncogenic transformation. The isoforms of CD45 correlate to specific cell types, as well as specific activation and maturation states of immune cells. The A20 monoclonal antibody has been used extensively in adoptive cell transfer and bone marrow transplantation studies in mice, allowing for the differentiation of CD45.2- and CD45.1- expressing cells.