 Systemic lupus erythematosus SLE is an autoimmune disease in which the immune system produces antibodies against the bodys healthy cells and tissues. These antibodies, called autoantibodies, contribute to the inflammation of various parts of the body and can cause damage to organs and tissues. Mixed connective tissue disease MCTD is another autoimmune disease that overlaps in terms of signs and symptoms with three other connective tissue diseases, including SLE. In both SLE and MCTD, the immune system appears to be abnormally activated by small nuclear ribonucleoprotein snRNP autoantigens. Furthermore, lung tissue, in particular, appears to be affected by the immune response induced by snRNP autoantigens. The causes of SLE and MCTD remain unknown. However, it is likely that a combination of genetic, environmental, and possibly hormonal factors work together to cause the diseases. Past studies suggest that several different genes may be involved in determining a persons likelihood of developing SLE or MCTD, which tissues and organs are affected, and the severity of the disease. The purpose of this study is to characterize immune system abnormalities, genetic components, and disease progression in people with SLE and MCTD. 