Nisman et al. detected significantly higher levels of two enzymes involved in tumor cells metabolism, pyruvate kinase isoenzyme type M2 (TuM2-PK), and thymidine kinase 1 (TK1), in RCC patients sera, compared to controls, and this increase is positively associated with tumor stage and/or grade [70]. Here, TK1 is linked to renal cell carcinoma.