ICAM1 and infection: Interestingly, the gene expression of an endothelial cell marker involved in leukocyte extravasation, intracellular adhesion molecule-1 (Icam-1), was significantly reduced in the brain tissues of Opn−/− mice at 2 d.p.i., suggesting defective leukocyte extravasation into the brain of Opn−/− mice at the early stage of infection (Fig. 3B).