Isoguanosine-5'-O-triphosphate, an isomer of guanosine 5'-triphosphate (GTP) and a phosphorylated derivative of crotonoside, does not promote microtubule assembly like GTP. At a concentration of 2 mM, it hampers transcription and triggers T to C mutations in reverse transcriptase assays. This compound aids in assessing the substrate specificity of both phosphofructokinase and mutT homolog 1 (MTH1).