Sir

Tsunoda and Kato1 report the successful cloning of mice after transferring cell nuclei from late preimplantation embryos. Our earlier studies2, using different methods, were mostly neglected or rejected. But Tsunoda and Kato found similar, although not identical, results concerning the ability of nuclei from the inner cell mass (ICM) of blastocysts to give rise to adult mice.

It is important to note that, under different experimental conditions, similar results have confirmed the developmental potential of ICM nuclei. Tsunoda and Kato also report the successful cloning of mice after transferring cell nuclei from the trophoblast of blastocysts, a type of cell thought to be specialized for its extra-embryonic development.

In contrast, McGrath and Solter3 concluded from negative results that the cloning of mice using donor nuclei from late preimplantation embryos was impossible, and suggested that the “inability of cell nuclei from these stages to support development reflects rapid loss of totipotency of the transferred nucleus”.

Now, with these positive data on the cloning of mice, the time has come for the correct evaluation of our earlier results on the first cloning of a mammal.

It has been argued that the early activation of the mouse embryonic genome is a barrier to cloning, leaving too little time for the nucleus to be reprogrammed. But even Solter4 now admits that this is not insurmountable, in the light of successful experiments by Wakayama et al.5 using adult cumulus cells for the cloning of mice.