TACSTD2 and lung cancer: Trophoblast cell surface antigen 2 (Trop2), also known as human tumor-associated calcium signal transducer 2 (Tacstd2), is a surface glycoprotein originally identified in human placental trophoblasts (7, 8), which is seldom expressed in normal tissues but highly expressed in various cancer cells, such as pancreatic cancer (9), gastric cancer (10), lung cancer, and colorectal cancer (11–13).