We show that the lack of CD84 in platelets does not affect classical platelet functions such as integrin activation, granule release and aggregation in response to major agonists or spreading in vitro. CD84 expression in platelets has been reported in earlier studies, but its role in platelet activation and thrombus formation has been elusive. A previous study revealed that CD84 undergoes tyrosine phosphorylation upon platelet activation and aggregation. One of the two phosphorylated cytoplasmic tyrosines was found in an ITSM, which is a putative recognition motif for the adapter proteins SAP and EAT-2. The requirement of SAP for platelet spreading on immobilized CD84 implicates a functional relevance of this signaling pathway. Interestingly, activation-induced tyrosine phosphorylation of CD84 was abolished when platelet aggregation was blocked with an aIIbb3 inhibitor. This aggregation-dependent phosphorylation was also observed for the other prominent SLAM family member on platelets, CD150. The subsequent analysis of CD150-deficient female mice further revealed a delay in thrombus formation in a FeCl3-induced thrombosis model in mesenteric arteries and weaker aggregation in response to collagen and a thrombin LEE011 receptor activating peptide. Due to these results CD150 and CD84 were proposed as thrombus stabilizing receptors in response to platelet aggregation but CD84-deficient mice were not available at that time to confirm this. However, a potential thrombus-stabilizing function of SLAM family members in platelets was further supported by the cooperation of CD84 and Ly108 in the stabilization of T cell:B cell contacts. In contrast, homophilic interaction of CD84 has been shown to negatively regulate FceRIITAM signaling in mast cells, which was found to be independent of SAP and EAT-2, but dependent on the inhibitory kinase Fes. Hence, also a negative regulatory role for CD84 in thrombus formation would have been conceivable. Similarly, platelet spreading and clot retraction were unaltered, demonstrating that CD84 is not essential for actin rearrangements in murine platelets. It is conceivable that there is a potential redundancy between platelet adhesion receptors and that the lack of CD84 may be fully compensated. Indeed, a wide range of other receptors have been reported or implicated to modulate platelet-platelet interactions such as ehrins/Eph-kinases, JAMs, CD150, or SEM4-D. Additionally, soluble mediators are involved in the stabilization of thrombi. On the other hand, our previous finding that CD84 is cleaved from the platelet surface upon platelet activation and aggregation suggests that CD84 may have a different function than stabilizing platelet-platelet contacts. Since besides platelets also many immune cell types abundantly express CD84 and because the receptor undergoes homophilic interactions, it appears possible that the receptor is of functional importance in platelet-immune cell rather than in platelet-platelet interactions. Shedding of CD84 would then provide a potential mechanism to regulate such interactions. However, this potential function of CD84 will be subject of future studies.