The efficiency of cloning, and the ability to generate healthy cloned animals, has been largely hampered by the difficulty of __________.

• A. inducing recombination in differentiated donor cells in order to restore the full genomic complement
• B. transforming donor cells with genes encoding proteins required for normal embryonic development
• C. physically removing the nucleus from the egg cell that will ultimately receive the donor cell nucleus
• D. implanting the clone into the surrogate mother
• E. completely reversing epigenetic alterations in donor cell nuclei such as DNA methylation and chromatin packing