Fig. 5.

Schematic representation of siRNA-mediated Xist repression and its effect on the survival of male SCNT-generated embryos. (Upper) In control cloned embryos, ectopic Xist expression increased rapidly from 48 h through 72 h and maintained a high level until 96 h (black line; see also ref. 6). Injection of Xist-siRNA into pronuclear (PN)-stage cloned embryos at 6 h (dotted arrow) resulted in repression of the Xist level over 48–72 h, but this became ineffective at 96 h (magenta line). Thereafter, around implantation, ectopic Xist expression diminished spontaneously from all embryonic tissues in Xist-siRNA and control embryos. (Lower) Repression of Xist by siRNA had a remarkable effect on the survival of cloned embryos. As early as E5.5, more than 10 times as many Xist-siRNA embryos survived compared with control embryos, and this high survival persisted until term. These results suggest that the adverse effects of ectopic Xist expression in cloned embryos are confined to a short critical time window in the preimplantation period and that this can be reversed very efficiently by injecting Xist-siRNA into SCNT-derived embryos.