BALB/c mice are susceptible to proteoglycan (PG) aggrecan-induced arthritis (PGIA), a murine model of rheumatoid arthritis (Glant,T.T. and Mikecz,K., Proteoglycan aggrecan-induced arthritis. A murine autoimmune model of rheumatoid arthritis. Methods Mol.Med. 2004. 102: 313-338.). However, there are marked differences among BALB/c colonies (maintained by different vendors at different locations) in PGIA onset and severity, which could be the result of subtle variations in their genetic background.