 Much of the debate about the ethics of cloning for research and therapy turns on a basic disagreement about how we should treat early human embryos. As it is currently done, the isolation of embryonic stem cells involves the destruction of embryos at the blastocyst stage (day five after fertilization, when the embryo consists of 125–225 cells). But cloning for research and therapy not only involves the destruction of embryos, it also involves the creation of embryos solely for the purpose of stem cell derivation. Views on whether and when it is permissible to create embryos solely to obtain stem cells differ profoundly.