Single-chain polypeptides of about 65 amino acids (7 kDa) from leeches that have a neutral hydrophobic N terminus, an acidic hydrophilic C terminus, and a compact, hydrophobic core region. Recombinant hirudins lack tyr-63 sulfation and are referred to as 'desulfato-hirudins'. They form a stable non-covalent complex with ALPHA-thrombin, thereby abolishing its ability to cleave fibrinogen.