During development the establishment of an axis is often followed by the division of the axis into repetitive series of similar but independent developmental units. 

Segments occurs in many species, from the obvious segmentation in the body of insects, to the rhombomeres and somites of vertebrate embryos. These segments can be considered as developmental compartments, in which the clonal expansion of a particular cell line is constrained. 

This may occur simply because there is a physical barrier to cell mixing, or compartments may be defined by patterns of gene expression in the absence of any obvious boundary. 

Homeotic genes give cells their positional identity. Different combinations of homeotic genes are expressed in response to different morphogen concentrations. 

The homeotic genes encode transcription factors that regulate downstream effector genes controlling differentiation and morphogenesis. 

Homeotic mutations cause cells to be assigned incorrect positional identities, resulting in the development of regionally inappropriate structures.