"Gene regulation is like a pyramid. At the
pyramid's apex are the master regulators, which
control a battery of secondary regulators, which
conerol the final products at the pyramid's base.
That is, some genes cause the synthesis of protein
molecules that control the expression of other
genes, which then cause the synthesis of the
protein molecules that actually make up the cell.
So once a developing cell makes the decision to
switch on a master regulator, that decision deter-
mines everything that happens subsequently "

The linked pdf is an article about brain
development.