Small number of genes trigger embryo development
Humans differ from mice in earliest days after fertilization, studies suggest
![a four-cell embryo](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2015/09/090815_ti_embryo_free.jpg?fit=860%2C460&ssl=1)
IN THE BEGINNING Early embryos (a four-cell embryo shown) from mice and humans look the same on the outside, but gene activity studies show some big differences under the hood.
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