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B7-H3 is a 316 amino acid-long type I transmembrane protein, existing in two isoforms determined by its extracellular domain. In humans it consists of one pair （2Ig-B7-H3） or two identical pairs （4Ig-B7-H3） due to exon duplication. B7-H3 mRNA is expressed in most normal tissues. In contrast, B7-H3 protein has a very limited expression on normal tissues because of its post-transcriptional regulation by microRNAs. However, B7-H3 protein is expressed at high frequency on many different cancer types （60% of all cancers）. In non-malignant tissues, B7-H3 has a predominantly inhibitory role in adaptive immunity, suppressing T cell activation and proliferation. In malignant tissues, B7-H3 is an immune checkpoint molecule that inhibits tumor antigen-specific immune responses. B7-H3 also possesses non-immunological pro-tumorigenic functions such as promoting migration, invasion, angiogenesis, chemoresistance, epithelial-to-mesenchymal transition, and affecting tumor cell metabolism.