FGFBP2 (fibroblast growth factor binding protein 2) serves as a marker for specific cytotoxic immune cell populations, particularly natural killer (NK) cells and certain T cell subsets, and plays important roles in cancer immunity and transplant rejection. The protein is highly expressed in NK cells and NK-like T cells, where it functions as a marker of cytotoxic activity 12. FGFBP2+ NK cells are involved in tumor immune surveillance, with their infiltration patterns correlating with treatment responses in various cancers including non-small cell lung cancer, acral melanoma, and hepatocellular carcinoma 123. The protein is also expressed in cytotoxic CD4+ T cells and memory T cells, where it marks terminally differentiated cytotoxic populations 45. In organ transplant rejection, FGFBP2 serves as an antibody-mediated rejection-selective transcript expressed in NK cells 6. Additionally, FGFBP2+ memory T cells appear to promote tumor metastasis, particularly in colorectal cancer, where their accumulation in metastatic sites correlates with angiogenesis 5. The protein's expression is associated with cytotoxicity-related genes and appears to be a reliable biomarker for immune cell activation states across multiple disease contexts.