Very few genetic differences are known between these substrains. The Qa-2 on Chromosome 17 is one gene that does differ between BALB/cJ and BALB/cByJ and involves a deletion in the BALB/cByJ genome (Mellor, A., et al., Proc. Natl. Acad. Sci., USA 82:5920, 1985). For additional variation among BALB/c substrains maintained throughout the world see The BALB/c Mouse. Genetics and Immunology, M. Potter (ed.), Current Topics in a Microbiology and Immunology 122, Springer-Verlag, 1985.
Both BALB/cJ and BALB/cByJ mice can be used to establish ascites tumors. In 1983, Dr. William J. Mandy at the University of Texas-Austin did a limited study that indicated hybridomas produced by fusing BALB/cJ spleen cells and SP2/o cells can be propagated as ascites tumors in BALB/cByJ mice. Thirty-three of 36 (91.6%) untreated BALB/cByJ mice inoculated i.p. with 106 hybridoma cells developed ascites tumors.
The major differences observed in the establishment of ascites tumors between BALB/cJ and BALB/cByJ were:
1. It took 4-5 days longer for the BALB/cByJ to develop observable tumor masses compared to the BALB/cJ. Tumors usually develop in 10-12 days in the BALB/cJ and 14-17 days in the BALB/cByJ.
2. The tumors that developed formed predominantly solid tumors, which may indicate that the BALB/cByJ strain tends to encapsulate the hybridoma lines more readily than the BALB/cJ strain.
3. After a secondary passage, the solid tumors in BALB/cByJ mice reverted to the ascites form; the antibody titers observed in the ascites fluid were comparable to that obtained from BALB/cJ.
BALB/cByJ has since been used successfully by others as an alternative to BALB/cJ for ascites tumor production. The BALB/cByJ substrain has the advantages of a higher reproductive performance and less aggressive behavior than BALB/cJ. These factors are reflected in the greater availability and the lower price of BALB/cByJ.