What the scientists did
Put a gene for a fluorescent protein into a single-celled human embryo to see if the fluorescent marker would carry into the daughter cells, allowing genetic changes to be traced as the cells divided.
Study author Zev Rosenwaks said that after three days, all the cells in the embryo glowed.
Why critics are concerned
They say the techniques being developed could be used by others to create babies with genes modified to make them smarter, taller, more athletic or better looking.
Some people find that notion repugnant, saying it turns children into designed objects, and would create an unequal society divided between the genetically enriched and others who would be considered inferior.