A number of inbred strains, various crosses, and recombinant inbred lines of mice spontaneously develop autoimmune manifestations of which resemble those of patients with systemic lupus erythematosus and to some extent, Sjogren’s syndrome and rheumatoid arthritis. Although several laboratories have characterized many of the immune abnormalities that occur secondary to genetic traits that segregate as Mendelian units, the primary defects have been elusive. Single gene defects which lead to or accelerate autoimmune disease include lpr, an unmapped autosomal recessive mutation (Murphy and Roths, 1978; Andrews et al, 1978) Yaa, the Y-chromosome-linked accelerator factor in BXSB mice (Murphy and Roths, 1979) and gld (generalized lymphoproliferative disease), a spontaeous autosomal mutation mapped to distal mouse chromosome 1 (Roths et al, 1984).