Which selectin is not found on endothelium?

L-selectin is primarily expressed on leukocytes rather than on endothelial cells. It plays a crucial role in the initial stages of leukocyte adhesion and rolling along the vascular endothelium but does not appear on the surface of endothelial cells themselves.

In contrast, both P-selectin and E-selectin are found on endothelial cells. P-selectin is stored in granules of activated platelets and endothelial cells and is translocated to the cell surface in response to inflammation. E-selectin is expressed specifically on activated endothelial cells and plays a critical role in mediating the interactions between the endothelium and circulating white blood cells, particularly during inflammatory responses.

Thus, the distinction lies in the expression patterns of these selectins, leading to the conclusion that L-selectin is the correct answer, as it is not found on the endothelium.