Abstract
Some evidence suggests that a number of regulator genes and gene clusters will likely be found to share with HOX complexes the property of being repressible ('superrepressible') through factor-driven conformational changes over whole sectors of chromatin, and of being assigned body locations in which they are either stably superrepressed or poised for transcription, according to determinants that act vectorially across a morphological zone. Such a subpopulation of regulator genes is expected to include, notably, genes governing developmental processes and might be thought to number, in mammals, between one hundred and several hundreds. When superrepressed, regulator genes are anticipated either to block programs of gene action or to permit these programs to unfold. To a significant extent, development would be determined by successive intersections of the paths of gene action deployment with superrepressed genes. These intersections, in cell lines advancing toward terminal differentiation, would be responsible for the progressive narrowing of the range of gene action programs potentially still available for later development. One implication of this model is that mosaic and regulative embryos are distinct merely by virtue of the time of onset of superrepression in their different cell lineages. Determination and transdetermination are considered to express the differential distribution over the genome of bound regulatory factors that function as molecular tools of superrepression, notably polycomb-group-like proteins. In turn, superrepressed genes are anticipated to be differentially distributed over cell types and thus to furnish a major framework for progressive differentiation and for the progressive limitation of the developmental potential of cells.
