Fertile offspring have been produced by nuclear transfer from adult somatic cells in several mammalian species (Wilmut et al., 1997; Kato et al., 1998; Wakayama et al., 1998; Polejaeva et al., 2000; Chesne et al., 2002; Shin et al., 2002; Zhou et al., 2003). Various possible causes have been suggested for the overall low efficiency (Perry and Wakayama, 2002). Notably, however, it has not yet been clearly demonstrated whether reprogramming after nuclear transfer is necessary for successful cloning. Here we show that reprogramming is essential in nuclear transfer, by comparing the developmental efficiency after the transfer of cumulus cell nuclei with that for zygote nuclei. Nuclear transfers from blastomeres of a series of pre-implantation stages showed further that, as development proceeds, the nuclei progressively lose their potency and become more difficult to reprogram upon their transfer into enucleated MII oocytes. We also found that naturally ovulated oocytes are much better recipients of a nucleus than are superovulated oocytes, which have been used in all the nuclear transfer experiments reported so far. This indicates that cloning efficiency can also be increased to some extent by technical improvements. All these results enable us to distinguish more clearly between the inherent problem of reprogramming and technical problems associated with materials, manipulation, and in vitro culture.