This study aimed to evaluate subcellular APEX1 expression in cancer cells and the extracellular APEX1 level in serum as a possible diagnostic or prognostic biomarker in patients with clear cell renal cell carcinoma (ccRCC), HCC, or cholangiocarcioma (CC), compared to the levels in nontumor groups, including a healthy control group and patients with ≥58 copies/mL of hepatitis B viral DNA (HBV DNA (+)). Here, APEX1 is linked to nonpapillary renal cell carcinoma.