The Molecular Biology of Smell 
receptor genes as molecular probes suggest a 
model of olfactory processing in which sensory 
neurons expressing distinct odorant receptors all 
project to a restricted region within the olfactory 
bulb, the first relay station in the brain. In this 
instance the discrimination of odors would be a 
consequence of the position of second-order neu- 
rons in the olfactory bulb, such that spatially lo- 
calized groups of neurons would preferentially 
respond to different odorants. The molecular 
identification of the genes encoding a large fam- 
ily of olfactory receptors has provided initial in- 
sight into the logic of olfactory processing in the 
mammalian brain. 
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