Among the four major groups of cell adhesion molecules (CAMs) the selectins represent the smallest and most recently identified gene family (Lasky 1992; Vestweber 1992). In contrast to the three large families of the integrins, the immunoglobulin super gene family and the cadherins, the selectins consist of only three members: L-, E-, and P-selectin. All three of them were originally identified by very different approaches and only by cloning and sequencing, in 1989, did it become evident that a new family of closely related CAMs had been discovered (Siegelman et al. 1989; Lasky etal. 1989; Bevilacqua etal. 1989; Johnston etal. 1989). Compared to the other three groups of CAMs, the selectins are unique in two respects: first, all three selectins are exclusively involved in the binding of leukocytes (and some metastatic cells) to endothelial cells which form the inner lining of blood vessels. No function during the morphogenesis of an organism has yet been described. Second, the selectins function as carbohydrate-binding proteins, in contrast to all members of the other three CAM families, which function via protein-protein interaction.