Systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) are long-term autoimmune diseases in which the immune system attacks parts of the body. The abnormal immune reaction causes inflammation of and damage to various body parts and can affect joints, skin, kidneys, heart, lungs, blood vessels, and the brain. SLE and MCTD often affect young women, especially black and Hispanic women, and there is no known cure. Knowing more about SLE and MCTD will help in developing new and effective treatments. The purpose of this study is to characterize immune system abnormalities, genetic components, and disease progression in people with SLE and MCTD.