The N-terminal domains of the cell-expressed CEA family of molecules are highly homologous and the majority of CEACAMs are targeted by one or more pathogenic neisserial adhesins belonging to the Opa family of proteins (Virji et al., 1996a; Chen et al., 1997; Gray-Owen et al., 1997) . Despite the structural variability, all Opa proteins target a common site on the receptors whose centre of binding appears to be Ile-91 on the CFG face of the N-domains. Ile-91 is conserved throughout the CEA members. Most of the other important residues on CEACAM1 identified by alanine scanning mutagenesis located in a close proximity of Ile-91 (Virji et al., 1999) . Precisely how variant Opa proteins can bind to a common receptor site is not entirely clear but a complementary set of sequences of more than one variable domains of Opa proteins may be involved. This variability of the ligands may determine the binding preference for distinct members of the CEA family which represents one mechanism that may determine tissue tropism (Virji et al., 1999; Bos et al., 2002; Gray-Owen, 2003; Hauck and Meyer, 2003; de Jonge et al., 2003) .