High amino acid sequence variability is found in the CFG faces of mouse PSG N1 domains. Alignments of the mouse PSG N1 domain amino acid sequences were performed using ClustalW. The locations of the β-strands (A-G) were derived from the crystal structure of the mouse CEACAM1 N domain , and are indicated by blue arrows. The boxed amino acids sequences form the CFG face of the N domain (deduced by structural modelling). (A) Alignment of mouse PSG N1 domain amino acid sequences. The signal peptide (leader) cleavage site is shown as a dotted line and N domain amino acid numbering commences from the first amino acid of the mature N domain. (B) Alignment of CEACAM N domains (minus signal sequences) showing all N domain interactions with pathogens and known binding partners (referenced as follows: 1 ; 2 ; 3 ; 4 ; 5 ; 6 ; 7 ; 8 ).