Komura, et al. (1975), first described a T cell antigen and classified it as Ly5.1 in C57BL/6 mice and Ly5.2 in SJL mice. To follow gene nomenclature, these designations became Ly5a for the B6 and Ly5b for the SJL.
In 1987, Morse et al proposed a change in murine lymphocyte antigen names, which led to a reversal of the previous designation. Thus, beginning in 1987, the B6 genotype became Ly5b (Ly5.2 antigen specificity) and the SJL became Ly5a (Ly5.1).
An additional change was made in 1992, when the gene name changed to Cd45, leading to SJL designation as Cd45a (Cd45.1) and B6 designation as Cd45b (Cd45.2).
Finally, the gene name was changed to Ptprc in 1993, after work by Charbonneau et al. demonstrated homology between a tyrosine phosphatase and CD45 protein. This led to the current designations of the B6 as Ptprcb and the SJL as Ptprca.
In addition to the SJL, the Ptprca (Ly5.1) allele is also carried by the STS/A and DA strains. The Ptprcb (Ly5.2) allele is expressed by the C3H, DBA/2, and AKR strains as well as the C57BL/6.
Taconic Biosciences offers a B6.SJL-Ptprca/BoAiTac congenic mouse which carries the SJL allele for Ptprca. The origin of the model states that Ly5.2 specificity was selected for as the congenic was bred. That statement reflects the nomenclature in existence at the time the model was developed in 1975. It should be noted that by current allele designations, this congenic model carries Ly5.1 (Cd45.1) specificity as conferred by the Ptprca allele from the SJL.