Certain classes of pathogenic bacteria secrete virulence proteins in a Sec-independent manner, by a mechanism known as type III secretion. The main body of the export apparatus specific for virulence proteins is identified as a needle complex, which has a similar structural organization to flagella. The two structures share several proteins with highly homologous amino acid sequences. Even where the sequence identity is low among flagellar proteins from various species, the physico-chemical properties of each protein remain homologous. Therefore, by comparing the physico-chemical properties of unidentified proteins, it is possible to find homologs among type III secretion systems.