CASA/RkJ (000735) and CAST/EiJ (000928) mice are inbred strains derived from a wild population of the subspecies Mus musculus castaneus. The strains are related but are not sublines of the same strain simply maintained by two different holders. M.m. castaneus mice trapped in Thailand by Dr. Joe Marshall were sent to Dr. Verne Chapman at Roswell Park Memorial Institute and then to Drs. E. M. Eicher and T. H. Roderick at The Jackson Laboratory in 1971.
CASA/RkJ and CAST/EiJ were inbred independently by Roderick (Rk) and Eicher (Ei) from this small, imported founder population. Because the mice imported were not inbred, they were segregating for more than one allele at many different loci. Either allele at each locus could have been fixed in either strain during inbreeding. Therefore, we can expect that CASA/RkJ and CAST/EiJ differ at many loci, and an allele typed in one strain should not be assumed to be the same in the other. At some loci, especially biochemical polymorphic loci, CASA/RkJ or CAST/EiJ may have the same allele as common inbred strains. Information on the loci typed for each strain and at which loci they are known to differ or be the same is available from the Mouse Phenome Database.
We are encouraging the use of CAST/EiJ because these mice breed better than CASA/RkJ. We also maintain a larger colony of CAST/EiJ, making it possible to maintain the colony more efficiently and fill requests rapidly. We plan to remove the small colony of CASA/Rk from the breeding colony if the strain can be successfully preserved as frozen embryos.