Figure 2.

Diagram of a region of human chromosome 17 that codes for two olfactory receptors. This figure, based on the work of Glusman et al. (46), illustrates one of many sequenced regions of chromosomes that code for olfactory receptors and also contain numerous mobile elements. Note the pattern of elements near the upstream control elements of the two olfactory receptor-coding regions (OR228 and OR 40). See the original publication for more details of this work. We hypothesize that some of these elements are used as genetic switches for the control of the expression of the 1,000 or more olfactory receptor genes. The mobile element-related and transposase mechanisms used could be evolutionarily related to those now known to control the expression of genes in the immune system. This would help explain how the olfactory system, like the immune system, expresses only one receptor gene in each stem cell. In both the olfactory and immune systems, such committed stem cells not only remember which receptor gene to express but also which one of the two alleles (30).