Illustration of the US biologist Richard Axel (born 1946), who is best known for his work on the olfactory (smell) system. Axel and his colleague Linda Buck discovered over one thousand rat genes that code for odorant receptors. These are proteins found on the surface of olfactory receptor cells located at the back of the nasal cavity. They showed that each receptor cell carries only one type of receptor protein, that recognises only a small number of odorant molecules. Binding of the odorant to the receptor sends an electrical signal to the olfactory bulb of the brain where the information is combined and interpreted as a particular odour. Axel and Buck received the 2004 Nobel Prize in Physiology or Medicine for their work.