Super-resolution imaging has revealed that a protein complex called cohesin helps keep certain active regions of the genome from blending together. The human genome is incredibly long — about two meters — but it must fit inside a cell nucleus that's only about 10 micrometers in diameter. To achieve this, DNA is wrapped around proteins called histones, forming structures known as nucleosomes. These nucleosomes are then organized into a structure called chromatin, which can be in two general forms: euchromatin, which is more open and active, and heterochromatin, which is more compact and less active. A recent study, led by Kazuhiro Maeshima from the National Institute of Genetics, ROIS, and SOKENDAI, challenges the traditional view of euchromatin as being consistently open and unstructured. Published in Nature Genetics on September 8, 2026, the study found that euchromatin can form dynamic, condensed domains in living human cells. Using advanced imaging techniques, including single-nucleosome tracking and super-resolution 3D-structured illumination microscopy (3D-SIM), researchers were able to visualize these domains at a resolution of about 100 nanometers — much finer than traditional methods. The study found that cohesin, a ring-shaped protein complex known for its role in organizing the genome, plays a key role in keeping these euchromatic domains from mixing with one another. When cohesin was removed from the cells, the nucleosomes within these domains became more mobile, making the chromatin more fluid-like. This increased fluidity allowed neighboring condensed domains to mix locally, which could potentially disrupt gene regulation. The findings suggest that cohesin acts as a physical barrier, helping to maintain the separation of active chromatin regions. This separation is important for proper gene regulation, as it prevents neighboring genes from interfering with each other. The study offers a new perspective on euchromatin, showing that it can be both condensed and dynamic, with its structure actively controlled. This new understanding may lead to deeper insights into how genes are regulated, how the genome is organized, and how these processes might go awry in diseases.