Endogenous transcription occurs at the 1-cell stage in the mouse embryo.

In most animal species, the earliest stages of embryogenesis are regulated by maternally inherited components, at least until the activation of the zygotic genome. Although the first proteins derived from the activation of the embryonic genome have been detected long ago at the 2-cell stage in the mouse, the exact timing of transcriptional activity resumption after fertilization is still a matter of debate. Any new information about this critical event is relevant to the practice of transgenesis and cloning of embryos as well as to the general understanding of the regulation of nuclear processes following fertilization. Using a new fluorescent method allowing the detection of in vivo RNA synthesis, we show that endogenous transcription by RNA polymerase II takes place unambiguously as early as at the late 1-cell stage. Furthermore, we demonstrate that transcription is first initiated in the paternal pronucleus.