CD45 is a receptor-like transmembrane protein tyrosine phosphatase (RPTP) expressed on all nucleated hematopoietic cells that is required for signal transduction. In particular, CD45 has been shown to be an important proximal antigen receptor signalling molecule for optimal effector responses in T and B cells, as well as for lymphocyte development. Naive T cells predominantly express CD45RA+ isoforms and switch to the expression of CD45R0 upon activation. Most recently, the effects of dimerization of the intracytoplasmic domain of CD45 have suggested that CD45 is negatively regulated by dimerization. The biological relevance of this effect in vivo, however, is not known.