The bidirectional expression pattern of CTSF (downregulated in renal/gastric cancers vs. upregulated in cervical cancer/NSCLC brain metastases) positions it as a disease-specific biomarker: serum CTSF levels combined with imaging could enhance diagnostic accuracy for NSCLC brain metastases, while tissue CTSF expression may predict recurrence risk in clear-cell renal cell carcinoma patients. The gene discussed is CTSF; the disease is gastric cancer.