On Jan. 31, 2026, a research team from the Institute of Science Tokyo reported they became the first in the world to directly visualize sites of active transcription in cells inside a living mouse. The team focused on a fluorescent protein called a “mintbody,” which was developed from an antibody that binds specifically to the phosphate marker that appears only during active transcription.

Professor Hiroshi Kimura at Institute of Science Tokyo (Science Tokyo) and his colleagues succeeded in creating a mouse that expresses this mintbody throughout its entire body. Their study is published in the Journal of Molecular Biology.

Like countless lights flickering in a dark room, transcription is constantly taking place inside the cell nucleus. Using the mintbody mouse, the researchers observed hundreds to thousands of glowing dots—representing actively working RNA polymerase II—in nearly all tissues, including the brain, liver, and kidneys.

Interestingly, the number of glowing spots varied greatly depending on the cell type. For example, T cells, a type of immune cell involved in defending the body against viruses and abnormal cells, showed many bright signals.

In contrast, neutrophils, another type of immune cell, displayed far fewer. These differences clearly reflect how actively each cell type transcribes genetic information according to its role.

The researchers also observed that transcription is highly active in developing and differentiating cells, while it becomes much more stable in fully mature cells. In the testes, they were able to track dynamic transcriptional changes all the way to the stage where transcription nearly stops during sperm formation.

This technology provides a powerful new tool for understanding fundamental biological processes such as development and cell differentiation. By combining this mouse with disease models—such as those for cancer or aging—researchers can directly observe how transcription differs between healthy and diseased cells.