Scientists have reconstructed a detailed family tree of cells in a developing mouse embryo, tracing them back to the moment a fertilized egg first divided into two cells. The work, published October 8, 2026 in Science, relied on a technology called DNA Typewriter, which records each cell division directly into the genome. The researchers inserted a recording sequence into a fertilized mouse egg, and as the embryo grew, the system logged every division in order, creating a biological logbook of development.
The study covered the first 13.5 days of mouse embryonic development, a period when organs and distinct tissues form. Of 100 fertilized eggs initially used, 10 embryos were available for examination, and the team focused detailed analysis on one specimen, Embryo No. 3, because its recording system worked especially well. The approach overcomes limitations of earlier techniques, which could damage DNA or lose the order of past divisions.
Led by Jay Shendure of the University of Washington School of Medicine and Chengxiang Qiu of Dartmouth College, the team found that different cell types emerge on their own schedules during development. Because the DNA record is passed to daughter cells, matching sequences reveal which cells share ancestors. The researchers say this lineage information could eventually help explain what goes wrong in congenital malformations, neurodevelopmental conditions, and cancer. Co-first author Haedong Kim described the system as a typewriter that "types onto a cell's own DNA instead of paper," always adding characters in sequence without overwriting earlier entries. The technology was originally developed by Shendure and Junhong Choi, now at Sloan Kettering Cancer Center.