PECAM1 and triple-A syndrome: In addition, the expression levels of VSMC and endothelial cell marker proteins α-SMA and CD31, respectively, were reduced in the AAA area due to the disruption of both medial and endothelial layers of the aortic wall, while the level of fibroblast marker protein vimentin was similar between the normal and AAA areas (Fig. 1h, i).