CD304, also known as Neuropilin-1 (NRP-1) or BDCA-4, is a type I transmembrane glycoprotein that plays crucial roles in various biological processes. It was initially discovered on the cell membrane of plasmacytoid dendritic cells (pDCs) and is also expressed on thymocytes, regulatory T cells (Tregs), and endothelial cells. CD304 is involved in neuronal development, angiogenesis, and immunosuppression by interacting with plexins and VEGF receptors. In the field of hematology, CD304 is particularly useful as a marker for minimal residual disease detection in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) and has been shown to be highly expressed in certain hematological malignancies such as blastic plasmacytoid dendritic cell neoplasm (BPDCN) and acute myeloid leukemia (AML). Additionally, CD304 is used to distinguish between different subsets of Tregs, which is important for understanding immune regulation.