Olfaction is one of the most important types of sensory perception in mammals, and it is the foundation for the exceptional sensitivity necessary to differentiate environmental and sexual signals. As a result, olfactory receptor (OR) genes are the largest gene superfamily, accounting for 6% of proteincoding genes in a typical mammalian genome (total OR genes/ total number of protein-coding genes in dog: 1100/19,300). Olfaction is employed to variable degrees in many areas of life among vertebrates, including food detection, predator avoidance, and social communication. The olfactory receptor (OR) gene family is the biggest in the mammalian genome, with over 1,000 functional OR genes, each of which codes for a single OR and is expressed in the olfactory epithelium cells in a sequential order. The binding of smells to ORs causes a signaling cascade to occur in the olfactory bulb of the brain, which is mediated by a G-protein coupled receptor. Dogs and rats rely on scent to travel, forage, and communicate, but humans rely more on visual and aural signals. Most animals rely on their sense of smell to survive. It is used to discover food, avoid threats, identify partners and offspring, and mark territory. Olfactory receptors (ORs) expressed in the olfactory epithelium of the nasal cavity detect various odour molecules in the environment.

Figure 1: Scheme of the olfactory system. (Left) Scheme of olfactory sensory neuron projections. Olfactory sensory neurons transduce odor information via electrical signals that trigger neurotransmitter release in the olfactory bulb. Mucus secreted by Bowman's glands and sustentacular cells protect the olfactory epithelium's structure and maintain homeostasis. (Right) Scheme of the olfactory system according to the process of olfaction.