The use of embryonic stem cells is a very controversial topic. Embryotic stem cells come from the embryo of human and many people believe that destroying a fertilized egg, or embryo, is like killing a human being. Also, certain religious groups have particularly strong objections to the use of human embryos in scientific research.
An alternative method of deriving human ES cells is somatic nuclear transfer, or cloning, which bypasses the need to destroy an embryo produced naturally by the fusion of sperm and egg. The cloning of humans is extremely controversial. However, many mammal embryos have been cloned, including a primate embryo, and in theory, a human embryo could be cloned.