This fact, said Axel, provides the answer to the question of how it is that mammals can recognize a vast array of odors. When the genes are introduced into other cells, they are expressed as RNA, but the proteins seem to get stuck and are not expressed in functional form. Axel thinks a transport protein might be required. Until the receptors are expressed and studied outside the animals, nothing can be known about how odorous molecules actually interact with their receptors. To get at the answer, Axel's group first needed to address the diversity of receptor expression in individual neurons.
Neurons can express the receptor genes in either of two ways.
In one model, one neuron makes all receptors. Every time an odorant binds to its receptor, every neuron will send a signal. In the other model, one neuron makes only one receptor. The problem now becomes one of spatial segregation. Axel's group considered three ways by which neurons can be organized in space.