CD40, also known as TNFRSF5, is an approximately 32-35 kDa type I transmembrane glycoprotein member of the TNF receptor superfamily. The extracellular domain of rat CD40 shares 81% and 56% aa sequence identity with mouse and human CD40, respectively. Multiple rat CD40 splice variants have been reported including soluble isoforms. CD40 is expressed on the surface of B cells, dendritic cells, macrophages, monocytes and platelets, as well as endothelial and epithelial cells (1, 2). Interaction of CD40 with its ligand, CD40 Ligand, leads to the aggregation of CD40 molecules resulting in the initiation of intracellular signaling in both CD40 and CD40 Ligand expressing cells (3). CD40 ligation by CD40 Ligand promotes B cell activation and T cell-dependent humoral responses (4, 5). CD40 serves multiple functions in both hematopoietic and epithelial cancers and is a target for tumor immunotherapy (6, 7). Dysregulation of CD40/CD40 Ligand expression and interactions contributes to the immune deficiency associated with HIV infection and AIDS (8, 9). It is also implicated in the pathology of multiple cardiovascular diseases including atherosclerosis, atherothrombosis, and restenosis (10, 11).