DNA Typewriter reveals timing of cell fate commitment in embryos

A team of researchers has successfully reconstructed the cellular family tree of a developing mouse embryo using a new tracking technology. The study, published Thursday, Oct. 8, 2026, in Science, details how scientists recorded cell relationships from the initial split of the fertilized egg through 13.5 days of development. This period precedes the full gestation window of 19 to 21 days for mice.

How the DNA Typewriter Works

The method relies on a system called DNA Typewriter, invented by Dr. Jay Shendure and Junhong Choi. Shendure is a professor of genome sciences at the University of Washington School of Medicine and a Howard Hughes Medical Institute Investigator. Choi works at Sloan Kettering Cancer Center. The technology inserts a redesigned tape into the genome of a fertilized mouse egg, utilizing the animal's own DNA as a recording medium.

Haedong Kim, a co-first author and postdoctoral scientist at UW Medicine, explained the mechanism. "Think of it like an actual typewriter, except it types onto a cell's own DNA instead of paper," Kim said. Each time a cell divides, the system adds one new character to the sequence without overwriting previous entries. Because these characters are filled in strict order, the resulting sequence reveals the history of cell divisions.

Previous methods involved cutting DNA, which researchers noted could leave scars, damage cells, and erase earlier records. Those older techniques also lacked sequential ordering, forcing scientists to guess timelines after the fact. In contrast, the DNA Typewriter writes without fully severing the DNA strand, allowing for steady, high-resolution recording over longer periods.

Study Findings and Cell Fate Timelines

The research team attempted this recording process in 100 fertilized mouse eggs. They ultimately examined 10 embryos, focusing their detailed analysis on Embryo No. 3 because its recording system was clearly active and had captured the most comprehensive history. A key insight emerged immediately after the fertilized egg split into two cells. The system created a permanent mark distinguishing these first two cells, enabling researchers to trace nearly every profiled cell back to one of the two founders.

While one founding cell produced more descendants than the other, both generated diverse cell types at almost equal ratios. The recordings allowed scientists to pinpoint exactly when specific cell types diverged onto separate developmental paths. Blood cells and retinal cells committed to their fates relatively early in development. Conversely, the outer layer of skin did not commit until noticeably later.

"Cell fate wasn't locked in all at once. Different cell types settled into their identity on their own separate schedules, some early and some late," the scientists stated. These findings align with decades of prior research but were measured within a single mouse subject.

Implications for Disease and Engineering

Dr. Chengxiang Qiu, a molecular and systems biologist at Dartmouth College in New Hampshire, served as co-senior author alongside Shendure. Other co-first authors included Qi Yu and Sophie Seidel, both affiliated with the UW School of Medicine’s Department of Genome Sciences and the Seattle Hub for Synthetic Biology. Shendure also leads the scientific directorship at the Brotman Baty Institute for Precision Medicine and the Allen Institute for Cell Lineage Tracing.

Kim emphasized the broader utility of this large-scale developmental map. "This kind of large-scale developmental recording gives scientists a map of how one cell becomes a whole body - which helps explain how organs form correctly, how that process can go wrong in birth defects, and how abnormal cell growth drives cancer," he said. As stem cell engineering advances, such recording tools may help guide the creation of cells for therapeutic use.

The study addresses the challenge of tracing lineage in mammals, where the vast number and variety of cells complicate analysis. While similar feats were achieved in transparent roundworms with simple structures, mouse embryonic tracing with older technologies yielded only fragmentary views. The DNA Typewriter offers a clearer path to decoding normal development and understanding congenital malformations, neurodevelopmental conditions, genetic disorders, and cancer.