The gene which codes for the enzyme hypoxanthine-guanine phosphoribosyl transferase. It is a selectable marker because cells which have a defective version of this gene are resistant to poisoning by toxic purine derivatives which result from the metabolic pathway that the HGPRT enzyme catalyzes. (The purine derivatives are toxic because they incorporate into DNA as a result of the HGPRT enzyme's actions). Because the defective gene cannot produce the enzyme, no toxic purine derivatives are produced; the gene can therefore be selected for.