We aimed to test cytotoxicity of Cldn1‐binding CPE variants on thyroid and lung cancer cell lines, such as K1 cells (PTC) and Nthy‐ori 3‐1 cells (transformed from normal human thyroid follicular epithelial cells), PC‐9 (lung adenocarcinoma), and SK‐Mes‐1 (lung squamous‐cell carcinoma). The gene discussed is CLDN1; the disease is lung carcinoma.