Each odor-detecting neuron (referred to as olfactory sensory neuron from here on), chooses a single odorant receptor gene from a fairly large number of options that are split into class I (fish-like) and class II (terrestrial-specific) odorant receptors. This strict selectiveness of sensory neurons is in part due to enhancers (DNA sequences that enhance transcription of a gene when bound by specific protein), which remain poorly understood. Understanding enhancer functions is of great interest due to their importance in gene expression as well as evolution and disease. However, they have not been sufficiently studied because they cannot be easily predicted from DNA sequences or chromosome modifications nor can they be easily identified.