La (SS-B) is one of four autoantigens commonly referred to as extractable nuclear antigens (ENA). The other ENAs are SS-A/Ro, RNP, and Sm. Each ENA is composed of one or more proteins associated with small nuclear or cytoplasmic RNA species (snRNP) ranging in size from 80 to 350 nucleotides. Antibodies to ENA are common in patients with connective tissue diseases including lupus erythematosus (LE), mixed connective tissue disease, Sjogren syndrome, scleroderma, and polymyositis/dermatomyositis. Anti-La antibodies are found in 15-40% of Sjogren's patients and 10-10% of SLE patients and is testing for anti-La(SS-B) is useful for the differential diagnosis of SLE and Sjogren syndrome.