Cloning confusion

Every cloning pronouncement has one guaranteed ingredient: controversy. This time, Atsuo Ogura (National Institute of Infectious Diseases, Tokyo, Japan) and Rudolf Jaenisch (Massachusetts Institute of Technology, Cambridge, MA) are duking it out. Ogura claims that most of the defects that Jaenisch has seen in his cloned mice were caused by the genetic instability of the embryonic stem (ES) cells that Jaenisch used as a cloning source. In turn, Jaenisch calls Ogura's claims of faithful gene expression “ludicrous” and “unbelievable.” Let the cloning wars continue.

At stake is an explanation of the various defects seen in some cloned offspring. Jaenisch believes that a number of things may be going wrong, but initially he has focused his attention on a subset of developmental genes called imprinted genes. Jaenisch found that a number of these genes were...