The lysine-specific gingipain (Kgp) is a cysteine protease expressed on the bacterial cell surface of the oral pathogen, Porphyromonas gingivalis. Kgp forms part of a unique family of cysteine proteases (C25), which share little sequence identity with other known proteins, but are distinctly related to caspases. The 100 kDa C-terminal haemagglutinin/adhesin (HA) region of the gingipains are multi-domain complexes that direct the biological activity of the N-terminal C25 protease domain and are comprised of tandem repeats of domains that belong to the newly discovered protein family, the "cleaved adhesin" domain family. These 19 kDa cleaved adhesin domains share at least 30% sequence identity and are therefore, homologous [1]. Li et al. [2] designated the putative adhesin domains in Kgp as K1, K2 and K3 (K3* as the variant in strain 381). It is predicted that the structural differences and similarities between each of the domains and their organization within the gingipain ultimately determine the biological function; for example, how do various biovars of the gingipains contribute to the level of virulence for a particular strain of P. gingivalis. So far, the adhesin domains are known to bind to haemoglobin, human serum albumin and fibrinogen, each with different binding affinities.