CD45 is encoded by 3 alleles in mice, differentially expressed by various inbred strains. The Ly5 gene was originally described with the gene product LY5.1 expressed in C57bl/6 and Ly5.2 expressed in SJL strains (Komura et al. 1975), this was subsequently expanded to include a third allele encoding Ly5.3 (Shen et al. 1986). Further, in 1987 a reversal of nomenclature was instigated resulting in the allele in C57bl/6 becoming Ly5b encoding Ly5.2 and the allele in SJL mice becoming Ly5a encoding Ly5.1 (Morse et al. 1987). Further changes were made in 1992 with Ly5.1 becoming CD45.1 (SJL) and Ly5.2 becoming CD45.2 (C57bl/6). Finally, following work demonstrating homology between the CD45 and a receptor linked protein tyrosine phosphatase the CD45a gene was renamed Ptprca and CD45b renamed Ptprcb (Charbonneau et al. 1988; Zebedee et al. 1991). A number of different isoforms isoforms of CD45 are expressed on murine leucocytes depending on the pattern of altenative splicing of 3 exons termed A, B and C encoding regions of ~50aa located at the N terminal region of the extracellular portion of CD45. The restricted proteins are termed CD45R with a designation depending on the expressed codon product. (Birkeland et al. 1989).