Provided are fusion target-binding proteins comprising a target binding moiety, an intracellular signalling region and a domain that promotes synthesis of arginine or an arginine precursor. The domain may be an enzyme domain such as an argininosuccinate synthase (ASS-1) enzyme domain, or an ornithine transcarbamylase (OTC) enzyme domain. Also provided are cells comprising such a fusion target-binding protein (for example cells that express the fusion target-binding protein), and nucleic acids encoding such fusion target-binding proteins. The invention also provides fusion target-binding proteins comprising a target binding moiety, an intracellular signalling region and a domain that promotes synthesis of tryptophan or a tryptophan precursor. Pharmaceutical compositions, medical uses, and methods of treatment, all using the fusion target-binding proteins, cells, or nucleic acids are disclosed. The proteins, cells, nucleic acids and pharmaceutical compositions may be used in the prevention and/or treatment of cancer, such as neuroblastoma or acute myeloid leukaemia.