GAPDH and renal cell carcinoma: To further define the subcellular location of the increased STIP1 protein, and study whether tumor cells actively secrete STIP1 protein, we conducted two experiments and found First, After culturing RCC tumor cells were cultured in serum-free medium for 24 h, we observed that the culture media clearly contained STIP1 but no intracellular GAPDH protein (Figure 1A).