HPRT1 and neoplasm: PPIA showed |ΔCt| = 1.45 ± 0.17, and GAPDH and HPRT1 had the highest difference in |ΔCt| (on average) between normal and tumor samples: 2.42 ± 0.20 for GAPDH and 1.91 ± 0.21 for HPRT1.