This means that embryonic genes that are switched on early can have the effect of switching on others, etc. Epigenetic cascades importantly involve expression of genes that code for transcription factors and others that code for signalling molecules, including morphogens. Each stage of the cascade seems to organise an stage of development of each of the body's regions. The result is that development of each region seems to proceed by a refining of the previous stage, with the gradual appearance of finer and finer details of regional organisation. They behave like the segment polarity genes of the fruit fly's epigenetic cascade.
Different factors are responsible for regionalising the caudal neural tube into spinal cord.
All vertebrates appear to have homeotic genes similar to the fruit fly's. They are important for the roles they play in regional development. Mice that don't have a functioning homeotic gene called Lim-1 develop normally, except that they are literally headless. The same or very similar genes exist in fruit flies and vertebrates, although fruit flies have only one set and mammals have four. Each set consists of a sequence of genes that are located next to each other on the same chromosome.