rious effect on binding of some Nm Opa derivatives and the smaller Q89N apparently is better suited overall for binding by Hi and especially Hi-aeg strains. The data emphasize the extensive inter-and intraspecies ligand variations in this receptor targeting, perhaps with greater interstrain differences in Nm. Structural aspects of bacterial ligands that affect the receptor recognition can be considered for strain C751 Opa proteins whose structures are known (Fig. 9B ) (Hobbs et al., 1994) . The hypervariable loops HV1 and HV2 of Opa proteins have been implicated in the binding of CEACAMs (Virji et al., 1999; Bos et al., 2002; de Jonge et al., 2003) . Given that the combinations of HV regions of OpaA, B and D of strain C751 provide three distinct combinations (Fig. 10C) , this is consistent with the distinct patterns of targeting of the variant receptor molecules observed in the study. However, the proteins must all contain sufficient similarity to bind the hydrophobic region around I91.