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. Rhombomeres have also been shown to be able to affect the position that reticulospinal and branchiomotor neurons grow in. Each rhombomere can cause a repeated pattern of rhombomere-specific neurons, including reticulospinal neurons, many of which share properties such as mediolateral location. Reticulospinal neurons also occupy different areas in rhombomeres in different species. It was found in one study that reticular neurons in the lamprey hindbrain, which included isthmic, bulbar, and Mauthner cells, developed in conserved rhombomere-specific positions, similar to those in the zebrafish. In different species, however, reticulospinal neurons occupy different areas in rhombomeres. The trigeminal and facial motor nuclei were also shown to not correlate well with the rhombomere boundaries in the lamprey. Several studies have shown that the fibroblast growth factor (FGF) is secreted from the midbrain-rhombomere 1 boundary. These proteins instruct cell behavior in the surrounding neuroectoderm. However, the mechanism behind the integration of the signal and the actions that follow remains unclear. Studies have shown that the FGF receptors, or FGFRs act partially redundantly to support cell survival in the dorsal ectoderm, promote r1 tissue identity, and regulate the production of ventral neuronal populations, including the midbrain dopaminergic neurons. In mice, while mutations of the fgfr2 and fgfr3 did not interfere with the development of the midbrain and r1, mutation of fgfr1 caused midbrain and r1 defects. Studies have shown that the earliest serotonergic neurons were started in the anterior rhombomeres. The