CD45 or Leukocyte Common Antigen (LCA/T200) family is comprised of at least four isoforms (CD45RA, CD45RB, CD45RC, and CD45RO) of membrane glycoproteins with MW of 180kDa-220kDa. It consists of a large, heavily glycosylated, amino-terminal extracellular domain of 400-550 amino acids, a single transmembrane region and a large carboxy-terminal cytoplasmic domain of 700 amino acids. CD45/LCA is expressed on hematopoietic cell lines but absent on non-hematopoietic cell lines, normal and malignant non-hematopoietic tissues. The intracellular portion of CD45 proteins has phosphatase activity and is involved in regulation of transmembrane signals, functioning as a leukocyte membrane receptor capable of transducing signals via its phosphotyrosine phosphatase domains.