X-chromosomes inactivation in cloned mouse embryos

To study whether cloning resets the epigenetic differences between the two X chromosomes of a somatic female nucleus, we monitored X inactivation in c loned mouse embryos. Both X chromosomes were active during cleavage of clon ed embryos, followed by random X inactivation in the embryo proper. In the trophectoderm (TE), X inactivation was nonrandom with the inactivated X of the somatic donor being chosen for inactivation. When female embryonic stem cells with two active X chromosomes were used as donors, random X inactiva tion was seen in the TE and embryo. These results demonstrate that epigenet ic marks can be removed and reestablished on either X chromosome during clo ning. Our results also suggest that the epigenetic marks imposed on the X c hromosomes during gametogenesis, responsible for normal imprinted X inactiv ation in the TE, are functionally equivalent to the marks imposed on the ch romosomes during somatic X inactivation.