Guanosine 5’-[γ-thio]triphosphate (GTPγS) is a hydrolysis-resistant analog of GTP that binds to and activates G proteins in the presence of magnesium. GTPγS has been used to obtain crystal structures of GTPases in their active states, as well as measure the stability of GTPases in the presence of ligands. A radiolabeled form of GTPγS, [35S]GTPγS, is commonly used in binding assays to determine agonism or antagonism at G protein-coupled receptors (GPCRs).