The tetraspan family protein members are characterized by four predicted transmembrane domains and are thought to be involved in physiological processes such as tissue differentiation, immunological responses, and sperm-egg fusion. TSPAN9 has recently been identified as a platelet tetraspanin and a component of tetraspanin microdomains that include the collagen receptor GPVI (glycoprotein VI) and integrin alpha6beta1 but not the von Willebrand receptor GPIbalpha or the integrins alphaIIbbeta3 or alpha2beta1, suggesting that TSPAN9 may act to regulate platelet function in concert with other tetraspanins and their associated proteins.