Nuclear transfer in cattle: birth of cloned calves and estimation of blastomere totipotency in morulae used as a source of nuclei.

The birth of a clone of five bull calves, each produced by fusing a cell taken from the same young embryo with an oocyte whose own nucleus has been removed, opens the way to the use of these new animal models for research and animal selection. The embryonic cells came from a 31-cell morula. Our results indicate that most of the nuclei at this stage are totipotent which seems to no longer be the case for those taken from the cells of the inner cell mass at the blastocyst stage.