Evidence for gene conversion/recombination between putative paired carcinoembryonic antigen related cell adhesion molecule (CEACAM) receptor genes. The accumulation of nonsynonymous (red curve) and synonymous substitutions (green curve) along immunoglobulin (Ig) variable- and Ig constant-like domain exons of putative paired CEACAM1-like receptors were determined after manual removal of sequence gaps. The type of encoded signalling motif is indicated by the colour of the gene name: red, immunoreceptor tyrosine-based inhibition motif/immunoreceptor tyrosine-based signal motif; green, immunoreceptor tyrosine-based activation motif (ITAM); blue, ITAM-like motif. Stretches of codons with no or minimal accumulation of synonymous substitutions suggest recent recombination events. No such recent events are evident for the putative opossum paired receptors CEACAM114 and CEACAM111 and CEACAM112 and CEACAM111. For abbreviation of species names see Table 1.