The ligands of Neisseria (i.e. Opa proteins) and of Hi so far identified (i.e. P5 proteins) share similar beta-barrel structure with surface-exposed variable loops. The regions of P5 that may engage with the receptor have not been identified but those of Opa proteins were studied by mutagenesis of strain H44/76 (de Jonge et al., 2003) . This strain is related to strain MC58, one of the strains used in the current study. The studies of de Jonge et al. implicated G ¥ (I/V/l) ¥ (S/E/Q) as the key motifs of HV2 regions (Fig. 9 ) of meningococcal Opa proteins in receptor targeting. Together with this, an 99 ELK motif of the Opa HV1 region might be involved in the three dimensional presentation of the receptor-engaging residues of the bacterial ligand (de Jonge et al., 2003) . Within the strain C751, OpaB and OpaD proteins contain the motif GxLxS at positions 172-176 and 167-171, respectively, whereas Opa A contains a 168 PxIxN motif. In the HV1 region, OpaA and B contain 99 DLK whereas OpaD contains EDK. Studies in our laboratories are in progress to assess the precise variant C751 Opa and receptor residue pairs involved in mutual recognition.