Myticin is a cysteine-rich peptide produced in three isoforms, A, B and C, by Mytilus galloprovincialis, the Mediterranean mussel. Some isoforms show antibacterial activity against gram-positive bacteria, while others are additionally active against the fungus Fusarium oxysporum and a gram-negative bacterium, Escherichia coli D31. Myticin-prepro is the precursor peptide. The mature molecule, named myticin, consists of 40 residues, with four intramolecular disulfide bridges and a cysteine array in the primary structure different from that of previously characterized cysteine-rich antimicrobial peptides. The first 20 amino acids are a putative signal peptide, and the antimicrobial peptide sequence is a 36-residue C-terminal extension. Such a structure suggests that myticins are synthesized as prepro-proteins that are then processed by various proteolytic events before storage in the haemocytes as the active peptide. Myticin precursors are expressed mainly in the haemocytes. The family Mytilin has been merged into this family.