Before revision: Each mammalian olfactory sensory neuron stochastically expresses only one out of  thousands of olfactory receptor alleles and the molecular mechanism for this selection remains as one of the biggest puzzles in neurobiology.
After revision: Each mammalian olfactory sensory neuron stochastically expresses only one out of up to thousands of olfactory receptor alleles and the molecular mechanism for this selection remains as one of the biggest puzzles in neurobiology.