Cloning is highly inefficient.
Only a few percent of attempts lead to births, and many animals die soon after birth. Newborns are often large and they frequently have problems with breathing or blood circulation, and others may have kidney or brain malformations. The risk of such outcomes is one argument against cloning people. Scientists have suggested that the problem might lie in how a clone's genes are regulated. A chemical modification called methylation acts like a reversible on-off switch for individual genes.