HPRT Assay Kit provides an enzymatic tool for continuous spectrophotometric monitoring of HPRT activity in a convenient 96-well plate format. In the assay, HPRT activity is measured as a rate of production of IMP, which is oxidized by recombinant IMPDH enzyme with simultaneous reduction of NAD+ to NADH measurable by absorbance at 340nm.The assay is developed for measuring HPRT activity in vitro or in lysates of cells. For maximal accuracy, the assays with cell lysates are run with and without PRPP in parallel. The absorbance rate observed in the absence of PRPP is used as blank and is subtracted from the absorbance rate measured in the presence of PRPP.