Publications with shared reference: The quantitative determination of metabolites of 6-mercaptopurine in biological materials. VII. Chemical synthesis by phosphorylation of 6-thioguanosine 5′-monophosphate, 5′-diphosphate and 5′-triphosphate, and their purification and identification by reversed-phase/ion-pair high-performance liquid chromatography and by various enzymatic assays