In contrast to the PSG, the CEACAM subgroup of canine CEACAM1-related genes underwent a recent expansion similar to that found in primates and rodents. In general, IgSF members which are expressed by leucocytes are diverging rapidly within their extracellular domains. This results in an average amino acid identity of about 54% between human and mouse orthologs [1] . The conservation of the CEACAM1 N domain, most relevant for pathogen and cell-cell interactions [37] , lies within this range, with an amino acid identity of 56% (dog versus human) and 42% (dog versus mouse). This is consistent with the notion that CEACAM1 plays a key role in various functions of the immune system [8] . In contrast, between mouse and human CEACAM16, CEACAM18 and CEACAM19 orthologs, the N domain amino acid identities are higher ranging from 60% to 91%, which argues for non-immunological functions, especially for CEACAM16, which exhibits an overall amino acid sequence identity of about 90% ( Table 1 ).