Do you mean HOX genes?( i.e., The genes that contain the instructions for laying out the body plan during development).
I’m afraid don’t know much about the regulation of these genes, but a (very quick!) look at the literature suggests that scientists are still trying to figure out exactly how these genes get ‘switched on’ during mammalian embryogenesis, and what controls their correct order of expression. So it looks like at the moment the mechanism is not still fully understood.