Over the last decade, studies of the spatial architecture of chromosomes reveal they are divided into "topological domains" - sections of DNA that are in contact with each other more often than their neighbors in the sequence map of the genome. For example, imagine the sequence map of DNA as a long necklace of beads. When you wind the necklace around your hand, some beads that are far apart along the necklace are brought closer together. It is becoming apparent that the folding and packing of DNA into chromosomes to fit inside the nucleus of a cell is not just a matter of efficiency. Chromosomes are highly structured complexes of DNA and proteins organized to allow access for gene expression and DNA processing. Now, a new paper published in the journal Molecular Systems Biology reports how an international team has produced comprehensive 3D maps of the spatial organization of the mouse genome, from embryonic stem cells to fully developed neurons. The scientists, from Germany, Italy, Canada and the UK, believe that such maps will help pinpoint genes that are involved in hereditary diseases.