proteins in animals are encoded by homeobox genes, and further not all homeobox genes are necessarily associated with homeotic functions or (mutant) phenotypes. The concept of homeotic selectors was further elaborated or at least qualified by Michael Akam in a so-called "post-selector gene" model that incorporated additional findings and "walked back" the "orthodoxy" of selector-dependent stable binary switches. The concept of tissue compartments is deeply intertwined with the selector model of homeosis because the selector-mediated maintenance of cell fate can be restricted into different organizational units of an animal's body plan. In this context, newer insights into homeotic mechanisms were found by Albert Erives and colleagues by focusing on enhancer DNAs that are co-targeted by homeotic selectors and different combinations of developmental signals. This work identifies a protein biochemical difference between the transcription factors that function as homeotic selectors versus the transcription factors that function as effectors of developmental signaling pathways, such as the Notch signaling pathway and the BMP signaling pathway. This work proposes that homeotic selectors function to "license" enhancer DNAs in a restricted tissue compartment so that the enhancers are enabled to read-out developmental signals, which are then integrated via polyglutamine-mediated aggregation. Homeotic mechanisms in plants Like the complex multicellularity seen in animals, the multicellularity of land plants is developmentally organized into tissue and organ units via transcription factor genes with homeotic effects. Although plants have homeobox-containing genes, plant homeotic factors tend to possess MADS-box DNA binding domains. Animal genomes also possess a small number MADS-box factors. Thus, in the independent evolution of multicellularity in plants and animals, different eukaryotic transcription factor families were co-opted to serve homeotic functions. MADS-domain factors have been proposed to function as co-factors to more specialized factors and thereby help to determine organ identity. This has been proposed to correspond more closely to the interpretation of animal homeotics outlined by Michael Akam. See also Homeotic gene, includes most of the Hox genes Homeobox, a DNA segment found in Hox genes References Developmental genetics Evolution Gene expression Mutation