The 2D1 antibody reacts with all isoforms of CD45, a type I transmembrane glycoprotein expressed on the surface of hematopoietic stem and progenitor cells and mature blood cells, except mature erythrocytes, platelets, and plasma cells; expression of CD45 is lost during differentiation of these cell lineages. CD45 is a member of the protein tyrosine phosphatase family and functions in a number of immunoregulatory processes, including cell activation, growth, differentiation, and oncogenic transformation. The large cytoplasmic portion of CD45 contains two tyrosine phosphatase domains, one of which is enzymatically active, that are involved in modulating the function of intracellular substrates such as the Src kinases Lck and Fyn. Several isoforms of CD45 have been identified, the expression of which differs according to cell type and functional status. Alternative splicing of three exons (4, 5, and 6) encoding the extracellular RA, RB, and RC polypeptide sequences gives rise to up to 8 isoforms with molecular masses in the range of 180 – 240 kDa.