Expression level of candidate reference genes. We observed the six housekeeping genes in normal liver specimens(Normal), paired tumor and non-tumor liver specimens of HCCs with-metastasis (TM and PM), paired tumor and non-tumor liver specimens of HCCs without-metastasis (TNM and PNM). Values are given as real-time PCR cycle threshold numbers (Ct values). Boxes represent the lower and upper quartiles with medians. The arbitrary lines at Ct 18, 23 and 28 distinguish the groups of differently expressed housekeeping genes.