Hox gene
The Hox gene also has been shown to play a part in the formation of the cranial motor nerves. The fate of a rhombomere has been shown to be affected by differential expression of the Hox gene. With mutations in the Hox gene, the cranial motor nerves formed in different locations than normal or simply did not form altogether. One possibility for this was that the Hox gene was somehow involved in regionalization within the neural tube, and that expression of this particular gene correlated with the amount of migration that occurred. However, no correlation could be found. Many studies showed small amounts of correlation, but there were equal amounts showing a complete lack of correlation. The amount of correlation that occurred was not enough to draw a concrete conclusion from. This, however, could have happened because studies have only drawn data points from a limited window of time. Another possibility for this lack of correlation is that most studies were based on in situ hybridization, which only maps the location of transcripts rather than proteins. A third possibility is that studies focused on rhombomeres as landmarks, and therefore correlated expression domains to these. While studies were unable to show a relationship between closely related neurons from the rhombomeres and Hox gene expression, the Hox gene is still an important factor when it comes to specification of the neuronal phenotype. The Hox gene was expressed rostrocaudally in the same sequence that was physically within the chromosome and its transcription was regulated by retinoic acid. The Hox gene has been identified in all vertebrates, and the number of Hox genes expressed increases as the vertebrate species diverge farther away from the invertebrate species. Certain neuron groups relate to Hox gene expression. At the r4 level, Hoxb1 is suspected to bestow rhombomere 4 cell identity.