It takes three weeks for a mouse to develop from a fertilized egg into a newborn. In that time, there are four days that arguably matter more than all the rest.

Right around day number nine, the embryo undergoes rapid changes that form the mouse’s major organs from germ layer cells.

Researchers at the University of Washington mapped this period of organ development in mice using RNA sequencing tools and published the results Wednesday in the journal Nature. The resulting dataset will serve as a reference point for how developmental diseases work on a genetic level.