HPRT is the abbreviation for a protein called hypoxanthine-guanine phosphoribosyltransferase. This protein is also called an enzyme, because it has a special function.
HPRT is found in virtually all cells from most living things including plants, animals, and even some microorganisms. It catalyzes the transfer of the 5-phosphoribosyl group from phosphoribosyl-pyrophosphate to the 9 position of the hypoxanthine or guanine base in the presence of magnesium to form the respective nucleotide and pyrophosphate (see figures).
In humans, the enzyme functions as a combination of four identical subunits, and the configuration of the active site has been studied in detail.