Exploration of CTSF as a tumor suppressor gene in NSCLC by constructing gene–gene interaction network and PPI network demonstrated that besides potential functions of cysteine-type peptidase activity, cysteine-type endopeptidase activity, lysosomal lumen, and vacuolar lumen, CTSF also functions in antigen processing and presentation of exogenous peptide antigen via MHC class II, antigen processing and presentation, and adaptive immune response. The gene discussed is CTSF; the disease is neoplasm.