The present invention relates to a novel nucleic acid molecule encoding an amino acid sequence, which is capable of forming a cyclic structure. Cyclization may occur within a cell or cell membrane, or linear forms of the molecules may be circularised or partially circularised, in vitro using isolated enzyme systems or chemical means. The cyclised amino acid sequence is generally in the form of a stabilized folded structure such as a cyclic knotted peptide, polypeptide or protein or functional equivalent. The nucleic acid molecules and cyclic and linear peptides are useful inter alia in the generation of molecules having animal or plant therapeutic properties, as well as in a range of diagnostic, industrial and agricultural, including horticultural, applications. Of particular importance is the use of these molecules in the protection of plants, such as crop plants, from pest and/or pathogen infestation.