Olfactory receptors (ORs), the first dedicated molecules that odorants physically
interact with to arouse an olfactory sensation, constitute the largest gene family
in vertebrates and include around 900 genes in human, and 1500 in mouse.

While dog, like many other mammals, has a much keener olfactory potential
than human, only 21 canine OR genes have been described to date. In this study,
817 novel canine OR sequences were identified, and 640 have been characterized.
Of the 661 characterized OR sequences, representing half of the canine repertoire,
18% are predicted to be pseudogenes, versus 63% in human and 20% in mouse.

Phylogenetic analysis of 403 canine OR sequences identified 51 families, and RH-mapping
of 562 showed that they are distributed on 24 dog chromosomes, in 37 distinct regions.
Most of these regions constitute clusters of 2 to 124 closely linked genes.
The two largest clusters (124 and 109 OR genes) are located on CFA18 and 21.
They are orthologous to human clusters located on HSA 11q11-q13 and HSA11p15,
containing 174 and 115 ORs respectively. A refined study of the orthologous clusters
between dog and human indicate a strongly conserved genomic distribution, suggesting
that OR genes evolved from a common mammalian ancestral repertoire by successive
duplications.

In addition, the dog repertoire appears to have expanded relative to human, leading to the
emergence of specific canine OR genes.