Embryonic genome activation (EGA) is when an embryo begins transcribing genes from its own genome. It starts at different times in different species, and it is useful to distinguish the first detectable activity from the later, larger wave: current literature reports low-level transcription in one-cell human and mouse embryos, followed by a major wave at the four-to-eight-cell stages in humans and the two-cell stage in mice.
What embryonic genome activation means
Early development initially relies substantially on RNA and other molecules placed in the egg by the mother. EGA is the start of transcription from the embryo’s own genome. Researchers also use the term zygotic genome activation (ZGA) for this process.
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EGA is one part of the maternal-to-zygotic transition (MZT). The MZT is the broader, coordinated shift in developmental control: embryonic transcription increases while maternal products are remodeled or cleared, and the embryo’s chromatin and other cellular conditions change. The terms EGA and ZGA are sometimes used for the wider handoff, but MZT more clearly names the whole transition. Kojima, Hoppe and Giraldez’s 2025 review describes the cytoplasmic and nuclear reprogramming involved.
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When does EGA happen?
The answer depends on the species and on what counts as “activation”: the first detectable transcription or the larger major wave. A 2025 perspective by Maki Asami and Anthony C. F. Perry highlights evidence for transcription before the traditional major-wave milestones. The early activity does not replace the later wave; they are distinct phases.
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| Species | Early activity reported | Major wave commonly described |
|---|---|---|
| Mouse | Immediate EGA begins within four hours after fertilization, chiefly from the maternal genome during that interval, according to Asami and Perry’s 2025 perspective. | Two-cell stage. |
| Human | Significant but low-magnitude transcriptional upregulation has been reported in healthy one-cell embryos, according to the same perspective. | Four-to-eight-cell stages. |
| Zebrafish | Transcription appears after roughly 2–2.5 hours of development in the comparative review; this is not a human or mouse cell-stage equivalent. | not stated (Lee, Bonneau and Giraldez, 2014). |
The human and mouse early-stage findings are a recent interpretation of transcriptomic evidence, not terminology used uniformly by all researchers. Older descriptions that place human EGA at four to eight cells and mouse EGA at two cells generally refer to the prominent wave, so they need not contradict reports of low-level earlier transcription. For comparison across studies, check the species, developmental stage or elapsed time, whether the result concerns an early or major wave, and how transcription was measured. See Asami and Perry’s 2025 perspective and Lee, Bonneau and Giraldez’s comparative review.
What changes during the transition?
The shift is not a single switch with one universally established trigger. As embryonic transcription increases, chromatin becomes more permissive to transcription, cell-cycle conditions change, and maternal RNAs and other factors are progressively remodeled or cleared. Together, these changes allow the embryo’s genome to play an increasing role in directing development. Reviews of vertebrate genome activation describe this coordination among nuclear, chromatin and cell-cycle changes; see Lee and colleagues’ 2017 review.
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Why embryonic genome activation matters
As development proceeds, the embryo needs gene products made through its own transcription. Evidence for the importance of this activity comes in part from animal models: the comparative review reports that transcription inhibition can permit some early divisions yet prevent zebrafish and Xenopus embryos from gastrulating. Those findings explain why zygotic transcription matters for later development in these models; they are not direct evidence about the outcome for a particular human embryo or pregnancy.
How to interpret the reported gene count
Asami and colleagues report 1,777 mouse genes upregulated in their analysis of immediate EGA at the stated false-discovery threshold. This is a study-specific result, not a universal count of genes activated in every embryo or a general threshold for EGA. The figure appears in the 2025 perspective.
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