CD40 is a Type I Transmembrane Glycoprotein that belongs to the TNF Receptor Superfamily. CD40 is expressed in B cells, follicular dendritic cells, dendritic cells, activated monocytes, macrophages, endothelial cells, vascular smooth muscle cells, and several tumor cell lines. The extracellular domain of CD40 is characterized by Cysteine rich repeat regions. Interaction of CD40 with its ligand (CD40L) leads to aggregation of CD40 molecules, which in turn interact with cytoplasmic components to initiate signaling pathways. Several different TRAF proteins (adaptor proteins) have been identified to serves as mediators of the signal transduction.