Abstract: The introduction of immunotherapy has changed the traditional paradigms in cancer treatment and has demonstrated substantial clinical benefits across various malignancies. However, this progress is accompanied with emerging challenges, such as immune evasion and low response rates to immunotherapy. As an immunomodulatory molecule, zymogen granule protein 16 (ZG16) plays a crucial role in immunotherapy by inhibiting the binding of programmed death 1 (PD-1) to its ligand PD-L1, preventing immune evasion and restoring the immune system's ability to recognize and eliminate tumor cells. ZG16 is downregulated in multiple gastrointestinal tumors, and its regulatory mechanisms vary depending on the tumor type. ZG16 expression is closely associated with patient prognosis, and it has been identified as a favorable prognostic marker in colorectal cancer. This article provides a systematic overview of the structural characteristics, expression regulation mechanisms, and antitumor immune functions of ZG16.