P- and E-selectin, commonly referred to as the “endothelial” selectins, were initially described and characterized in the mid-late 1980s. P-selectin was first identified in 1984, using antibodies raised against activated platelets [1, 2]. In these initial publications, P-selectin was described as a protein of molecular weight of approximately 140 000, which was not found on resting platelets, but showed up-regulated expression following activation with thrombin or other mediators. In a subsequent investigation, Stenberg et al. [3] referred to this protein as “granule membrane protein-140” (GMP-140) due to its localization in the a granules of unstimulated platelets. In this study, as well as another report by Berman et al. [4], P-selectin was shown to translocate from the a granules in platelets to the plasma membrane following activation. In this latter publication, P-selectin was termed “platelet activation-dependent granule-external membrane protein” (PADGEM; see Table 1 for a listing of the other common abbreviations and designations for P-and E-selectin). In 1989, this adhesion molecule was shown to be expressed on the surface of cultured human endothelial cells following stimulation with histamine, thrombin, C5b-9, and other activators, and stored in resting cells in the Weibel-Palade bodies [5, 6, 7, 8]. During this same year, a cDNA for P-selectin was cloned, and sequence analysis suggested a cysteine-rich protein similar to that reported for a new endothelialexpressed protein termed ELAM-1 (endothelial leukocyte adhesion molecule-1; see below) [9, 10].