4-Ketobenzotriazine-O-CH2-COOH
2-[(4-Oxo-1,2,3-benzotriazin-3-yl)oxy]acetic acid (HBA) is a hapten with a carboxyl group at the end of its spacer arm, suitable for reacting with free amine groups of proteins. 2-[(4-Oxo-1,2,3-benzotriazin-3-yl)oxy]acetic acid can be combined with carrier proteins and used in antigen design.