Effects of ADP and some of the other relatively well understood thrombotic mediators on signaling in platelets. Most platelet agonists work using G protein–coupled receptors that act as guanine nucleotide exchange factors for 1 or more G proteins. An asterisk next to a receptor or G protein indicates that mice lacking that gene have been created but have not necessarily been studied for platelet function in the absence of the protein. PLC, phospholipase C.