Hypoxanthine phosphoribosyltransferase (HPRT, atmosphere of 5% C02/95% air. Methods for growing cells on plastic petri plates and for the selection of HPRT-deficient mutants or revertants have been previously described (12). When large quantities of cells are desired, they are grown in suspension at a density of S to 100 X 104/ml in Joklik's modified minimal essential medium (Gibco, 13.3 g of powder medium per liter) containing 5% calf serum (Irvine Scientific).Radioisotope-labeled amino acids are incorporated into HeLa proteins as previously described (12) buffer (10 mM MgCl2, 30 mM KC1, 0.1 mM dithiothreitol, 0.5% Triton X-100, and 10 mM Tris-HCl, pH 7.4) and centrifuged for 2 hr at 96,000 X g in a Spinco type 50 rotor. The high-speed supernatant fraction is removed, and to 2 ml supernatant are added 0.6 ml enzyme buffer (20 mM KC1, 6 mM MgCl2, 0.1 mM EDTA, 0.5 mM dithiothreitol, and 20 mM Tris-HCl, pH 7.8), 0.4 ml of a solution containing 10% Triton X-10(0 by volume and 10% sodium deoxycholate by weight, and 1.0 ml of 4 M NaCI. The sample is applied to 0.12 ml of