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"If two individual cells have a same mosaic variant, they were born from a common mother cell that passed it to all of its daughters," Yang explained.

The team found that the number of divisions a mother cell undertakes to restore its daughters to the correct starting size deviates from the mathematical optimum that was assumed to dictate this process.

The invading embryonic cells stimulated mother cells to make some immune cells that rein in immune responses, Teichmann and her team report today in Nature.

The team found levels of these “mother cells” dropped with age in the front and middle region of the dentate gyrus.

More than a decade ago, Clevers identified a type of mother cell in the gut that can give birth to all other intestinal cells.