The primary MR analysis identified seven significant plasma protein-SCLC pairs (p-value < 6.81 × 10−5; Figure 1B; and Table 1), including acid phosphatase 5 (ACP5), carboxypeptidase B2 (CPB2), glutathione S-transferase Mu 3 (GSTM3), serine hydroxy-methyltransferase 1 (SHMT1), Cathepsin B (CTSB), Netrin G1 (NTNG1), and Family With Sequence Similarity 171 Member B (FAM171B). This evidence concerns the gene CTSB and small cell lung carcinoma.