A working hypothesis shows the differential expression of <i>Atoh1</i> in inner hair cells (IHC) and outer hair cells (OHC).

<p>Based on our <i>in situ</i> hybridization data, <i>Atoh1</i> is expressed earlier and at a higher level in IHCs (red line) than OHCs (green line), first in the base (top) followed by the apex (bottom). At early postnatal stages, <i>Atoh1</i> is downregulated to below the detection threshold of <i>in situ</i> hybridization, but a low level of <i>Atoh1</i> expression (grey lines) likely is sustained and can be detected by <i>Atoh1-LacZ</i> as late as P275 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030358#pone.0030358-Matei1" target="_blank">[14]</a>. In the CKO mutant cochlea, the <i>Atoh1</i> expression is reduced to a range of different levels shortly after its upregulation and is then eliminated after a few days (dashed lines). Some cells at the apex express <i>Atoh1</i> at a level close to control hair cells and can survive to adult stage.