Iterated embryo selection is a hypothetical technique that, if developed, could be used to select for embryos with specific traits more efficiently. First, several embryonic stem cells would be collected from different donors. These stem cells would then be differentiated into sperm and egg cells, and the sperm cells would fertilize the eggs. Then the genomes of the resulting zygotes would be sequenced, and the zygotes with a large number of desirable alleles would be kept, and the rest discarded. Then the remaining zygotes would be induced to differentiate into sperm and egg cells, and the process would be repeated with these sperm and egg cells until an acceptable zygote is obtained. The repetition simulates evolution on a much shorter time-scale, and allows greater control over the genome than it would be possible to get by selecting the most desirable zygote after one generation.