Female Chromosome 1 congenic mice (C57BL/6.129[D1Mit105–223]), together with sex-matched Apcs−/− mice backcrossed onto C57BL/6 for ten generations (C57BL/6.Apcs−/−) and C57BL/6 controls, were monitored for the presence of lupus. This evidence concerns the gene APCS and systemic lupus erythematosus.