Table 1.

Characteristics of olfactory neuron differentiation

The OR gene family of mice includes >1000 genes spread out over most chromosomes. About 85% of these can express functional receptors (21). In humans, only ∼40% are functional (22,23). The majority of all mature neurons express only one receptor (5,24,25). Expression of an OR protein enforce a feedback that (normally) keeps the neuron from expressing a second OR (24,26–28). Immature neurons can switch between expressing different ORs, including the two allelic versions, before maturation (24). At least 40% of neurons die before they fully mature (29,30). Pseudogenes are OR genes that do not result in a functional receptor and may not invoke the presumed feedback. Pseudogenes can be co-expressed with a functional OR (27,28). Transgene expression of an OR gene from a promoter associated with early transcription results in OR expression over the full OE (15). Enhancer elements upstream of an OR locus alter the probability of the associated OR genes being chosen for expression (25,27,31). Promoter and coding region of ORs contain TF-binding sites (13,32,33). However, identical transgenes are not co-expressed, and thus OR gene choice cannot be fully governed by TFs (5). Silenced OR genes are covered by nucleosomes marked with H3K9me3 and H4K20me3 methylation, whereas the nucleosomes associated to active OR genes are marked with H3K4me3 (34).