Nelmastobart is a human IgG4-kappa monoclonal antibody that specifically targets butyrophilin subfamily 1 member A1 (BTN1A1), a member of the butyrophilin (BTN) family of immune-modulatory proteins. As an IgG4 isotype, it is designed to minimize effector functions (e.g., antibody-dependent cellular cytotoxicity) and focus on modulating BTN1A1-mediated signaling. BTN1A1 is implicated in regulating T-cell responses and has been associated with certain cancers and autoimmune conditions. By binding to BTN1A1, nelmastobart aims to interfere with its immunosuppressive or regulatory activity, potentially restoring or enhancing anti-tumor immunity. It is being developed as an immuno-oncology therapeutic for the treatment of BTN1A1-expressing cancers.