Odorant binding proteins are abundant small soluble proteins secreted in the nasal mucus

Mucus

In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 of many animal species and in the sensillar lymph of chemosensory sensilla

Sensilla

A sensillum is an insect sensory organ protruding from the cuticle, or sometime lying within or beneath it. Sensilla are divided in chemical, mechanical , thermal and visual....

 of insects.

The olfactory receptors of terrestrial animals exist in an aqueous environment

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, yet detect odorants that are primarily hydrophobic. The aqueous solubility of hydrophobic odorants is greatly enhanced via odorant binding proteins which exist in the extracellular fluid surrounding the odorant receptors. This family is composed of pheromone binding proteins (PBP), which are male-specific and associate with pheromone-sensitive neurons and general-odorant binding proteins (GOBP).

These proteins were initially identified on the basis of their ability to bind with moderate affinity radioactively labeled odorants.