PECAM1 and neoplasm: Macroscopically, PAIS is always an intravascular mass which is closely adherent to the vascular intima, and can extend down into adjacent vessels.[4,26] Histologically, the tumor is commonly composed of spindle or epithelioid cells with various atypia; myxoid and fibrosis changes are common, as noted in our case.[4,27] Immunohistochemical staining for PAIS shows variable expression of desmin and smooth muscle actin but negative for epithelial markers, factor VIII, CD31, CD34, and S100.