Scientists at the University of Washington and the Seattle Hub for Synthetic Biology have reconstructed a lineage tree tracking 1.28 million cells in a developing mouse embryo. Published on October 8, 2026, in Science, the research uses a modified DNA recording technology called DNA Typewriter to log cell divisions over a 13.5-day period.
Every complex animal begins as a single fertilized egg, yet tracking how that founding cell multiplies into millions of specialized descendants has long challenged developmental biology. While researchers previously mapped the complete cell lineage of transparent roundworms in 1983, achieving the same resolution in a mammal has remained out of reach due to in utero development that hides tissues from direct microscopic observation.
To overcome this limitation, a research team led by Jay Shendure at the University of Washington School of Medicine and Howard Hughes Medical Institute reconstructed the family tree of cells in developing mouse embryos over a 13.5-day span, which covers approximately two-thirds of full gestation, while full gestation in mice lasts 19 to 21 days.
How DNA Typewriter Records Cell Divisions in Living Embryos
The milestone was achieved using a tracking technology called DNA Typewriter, originally invented by Shendure and Junhong Choi, who is at Sloan Kettering Cancer Center, to log three phrases in lab-cultured cells, including a verse from the Old Testament first transmitted wirelessly by Morse in 1844, the world’s first telephone currency by Bell in 1876, and a line from a BTS song. For the new mammalian study, the scientists inserted a newly redesigned synthetic DNA tape into the genome of fertilized mouse eggs before the first division.
“Every time a cell divides, it strikes one new character onto the next blank line — always in order, never overwriting what’s already there. Because the characters are always filled in sequence, that sequence itself spells out the cell’s division history.”
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Older methods that cut DNA can leave scars, prove harsh on cells, and quickly run out of recording capacity. By contrast, the team used a next-generation gene editing approach known as prime editing that cuts only one strand. This mechanism allowed the DNA Typewriter to add six-nucleotide DNA fragments in strict chronological order without severing the genetic material entirely, recording cell lineages at higher resolution for a longer duration.
Tracing 1.28 Million Cells Back to Two Founding Cells
In Embryo No. 3, the recording system gathered the richest history, prompting a full analysis that mapped 1.28 million individual cells—corresponding to roughly 10 percent of all cells in a two-week-old mouse.
A fortuitous mark captured immediately after the egg split in two allowed scientists to trace nearly every profiled cell back to one of the two original founding cells.
- One founding branch accounted for about 57 percent of the sampled cells.
- Despite this numerical imbalance, both branches produced diverse cell types at almost equal ratios.
This finding suggests that early developmental cell fate decisions operate independently of the total raw number of descendants each founding cell eventually produces according to the researchers.
Lineage Mapping Tools Help Research Developmental Disorders
The Science publication coincided with related research published by a team including HHMI Investigators Jonathan Weissman and Jay Shendure separately developing new tools, with Weissman’s group using a separate prime-editing tracking system called PEtracer. Both lineage-mapping efforts aim to provide foundational datasets for understanding organ formation and developmental disorders. Jay Shendure noted that exploring cell lineages helps decode normal development and could eventually advance knowledge regarding congenital malformations, neurodevelopmental conditions, genetic disorders, and cancer.
Co-senior author Chengxiang Qiu, a molecular and systems biologist at Dartmouth College in New Hampshire, collaborated on the University of Washington study. Beyond basic developmental biology, researchers suggest that recording lineage histories directly in DNA will help train artificial intelligence models of mammalian development and track how tumors initiate, spread, and develop resistance to therapies.