CD40 is a type I transmembrane glycoprotein belonging to the TNF receptor superfamily. The mature hCD40 consists of a 172 amino acid (aa) extracellular domain, a 22 aa transmembrane region and a 62 aa cytoplasmic domain . Human and mouse CD40 share 62% aa identity. 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.