Genetically engineered mice bring animal studies to the molecular level in that they help establish the role of a particular molecule, or its portion, in a complex biological process. In recent years, several discoveries were made using the selectin-mutant mice. For example, it was shown that these molecules not only mediate leukocyte rolling, but also platelet rolling on the vessel wall. The functional significance of platelet rolling has yet to be uncovered. The process could be important for hemostasis leading to firm platelet adhesion at sites of denuded endothelium and/or in inflammation. After activation, platelets may help in leukocyte recruitment as shown by studies of lymphocyte homing to peripheral lymph nodes. Surprisingly, work with the P-selectin mutant mice has also revealed an anti-inflammatory aspect of platelet P-selectin. P-selectin binding to leukocytes promoted the transcellular production of an anti-inflammatory mediator limiting the extent of acute glomeluronephritis. In addition, soluble P-selectin was shown recently to be shed from both activated platelets and endothelium and there are strong indications that it too could have an attenuating effect on inflammatory disease progression. Another discovery made with the selectin-deficient mice is on the crucial role of P- and E-selectins in the homing of hematopoietic progenitor cells to the bone marrow. This observation could perhaps be further exploited by use of selectin inhibitors when liberating the progenitors from the marrow for transplant purposes. The use of selectin inhibitors could also be evaluated in two major disease processes where selectins were recently shown to play a role: cancer and atherosclerosis. Thus the selectin mutant mice have taught us a great deal about the role of selectins in normal physiology and in pathology. Further studies are needed to explore the regulation of shedding of the selectins and the function of soluble selectins in vivo. Exploring new territories of selectin-mediated interactions may provide a basis for developing new interventions and treatments for diseases in which the role of selectins has not yet been suspected.