Indoleamine 2, 3-dioxygenase (IDO1), the first and rate-limiting enzyme of the KP, has also been investigated due to its role in the pathogenic process of atherosclerosis.22 IDO1 is expressed in a variety of cells, including endothelial cells,23–25 vascular smooth muscle cells, macrophages,26 leukocytes,27 and dendritic cells (DCs),28 all of which are present in the artery wall. This evidence concerns the gene IDO1 and atherosclerosis.