P-selectin is an adhesion receptor for leukocytes that is expressed on activated platelets and endothelial cells. The surface expression of P-selectin is tightly regulated through signals in the cytoplasmic domain that mediate sorting into secretory granules, internalization into coated pits, and targeting to lysosomes for degradation. Like the other selectins, P-selectin binds sialylated, fucosylated carbohydrate ligands such as sialyl Lewis x. However, it binds with much higher affinity to PSGL-1, a sialomucin-like glycoprotein on myeloid cells. Binding of PSGL-1 to P-selectin may be essential for leukocytes to roll on P-selectin-expressing cells under shear forces.