The aim of this study is to explore cytotoxic T lymphocyte associated antigen 4 (CTLA-4), programmed death 1 (PD-1) and programmed death ligand 1 (PD-L1) distribution and clinical value in liquid biopsy (such as blood) of small cell lung cancer (SCLC) patients. This evidence concerns the gene PDCD1 and small cell lung carcinoma.