<p>The precursor sequences of a number of antimicrobial peptides secreted by neutrophils (polymorphonuclear leukocytes) upon activation have been found to be evolutionarily related and are collectively known as cathelicidins [<cite idref="PUB00001695"/>].</p><p>Structurally, these proteins consist of three domains: a signal sequence, a conserved region of about 100 residues that contains four cysteines involved in two disulphide bonds, and a highly divergent C-terminal section of variable size. It is in this C-terminal section that the antibacterial peptides are found; they are proteolytically processed from their precursor by enzymes such as elastase. This structure is shown in the following schematic representation:</p><pre> +---+--------------------------------+--------------------+ |Sig| Propeptide C C C C | Antibacterial pep. | +---+----------------|--|--|--|------+--------------------+ | | | | +--+ +--+'C': conserved cysteine involved in a disulphide bond.</pre>