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Folate receptor 4 （FR4）, also known as folate receptor delta or IZUMO1R, is a protein encoded by the FOLR4 gene in humans. It is a subtype of the receptor for the vitamin folic acid and is primarily expressed on CD4 T lymphocytes, with lower expression on CD8 T cells. FR4 is highly expressed in regulatory T cells （Tregs） and is also found in tumor-associated macrophages （TAMs） and T follicular helper cells （TFH cells）. This protein plays a role in immune regulation, as its high expression can distinguish Tregs from other T cells. Additionally, FR4 is involved in the high-affinity binding and transport of folate and folate analogs across the cell membrane.