Systemic lupus erythematosus is an often devastating autoimmune disease which affects 1 in 2,000 women in the United States. Recently, several research laboratories have reported that a protein, named CD40-ligand (CD154), is overexpressed by a subset of white blood cells, called lymphocytes, in patients with lupus. Expression of CD154 appears critical to the generation of antibodies that cause disease in lupus. Blocking CD154 interactions in the immune system has been shown to decrease disease activity in animal models of lupus. We propose to study the regulation of CD154 in patients with lupus in hopes of inhibiting its abnormal and deleterious expression.