Combinations of a few gene regulatory proteins can generate many different cell types during development. In this simple scheme a "decision" to make a new gene regulatory protein (shown as a numbered circle) is made after each cell division. Repetition of this simple rule enables eight cell types (A through H) to be created using only three different regulatory proteins. Each of these hypothetical cell types would then express different genes, as dictated by the combination of gene regulatory proteins that are present within it.  Fair Use and Copyright info