Mammalian cloning: advances and limitations.

For many years, researchers cloning mammals experienced little success, but recent advances have led to the successful cloning of several mammalian species. However, cloning by the transfer of nuclei from adult cells is still a hit-and-miss procedure, and it is not clear what technical and biological factors underlie this. Our understanding of the molecular basis of reprogramming remains extremely limited and affects experimental approaches towards increasing the success rate of cloning. Given the future practical benefits that cloning can offer, the time has come to address what should be done to resolve this problem.