Abstract The tumor necrosis factor (TNF) region spans four loci: lymphotoxin a (LTA), lymphotoxin b (LTB), tumor necrosis factor (TNF), and leukocyte-specific transcript 1 (LST1). Much is known about the functions of the products of three of these genes—LTa, LTb, and TNF. The fourth gene has been relatively ignored since its discovery in the early 1990s. The most studied gene product is the cytokine is TNF (also known as TNF-a), which is produced largely by monocytes, macrophages, and T cells (Correa & Anaya, 2001). TNF is synthesized as a 26-kDa membrane protein, which is cleaved to produce its soluble 17-kDa form. TNF is a pleiotropic and proinflammatory cytokine that displays a wide range of biological activities regulating normal functions such as hematopoiesis, morphogenesis, lipid metabolism, and endothelial function and has several local effects, including protection against infections. However, TNF overproduction can lead to severe tissue damage, organ failure, and occasionally death.