The tetraspanin CD63 (orange, green, and blue molecule) is needed to organize P-selectin into small clusters to enable rolling. P-selectin (CD62P) lectin and EGF domains shown as ribbon diagrams based on crystal structure pdb 1G1R. Consensus repeats shown as turquoise ellipses (no structure available), transmembrane and cytoplasmic domain shown as putative α helix and unordered domain. Professional illustration by Marie Dauenheimer.