 However, what many people find disturbing is the idea of creating a genetic duplicate of an existing person, or a person who has existed. That is why the potential application of SCNT in humans set off a storm of controversy. Another way to produce a genetic duplicate from an existing person is by cryopreserving one of two genetically identical embryos created in vitro for several years or decades before using it to generate a pregnancy. Lastly, reproductive cloning of humans could, in theory, also be achieved by combining the induced pluripotent stem cell technique with tetraploid complementation. Several research teams have succeeded in cloning mice this way (see, for example, Boland et al. 2009). The technique involves injecting mouse iPS cells in tetraploid embryos, i.e. embryos with twice the normal number of chromosomes that cannot result in live offspring. The resulting mouse pups are derived solely from the iPS cells, which means that the tetraploid embryos only acted as a substitute trophectoderm, which forms the placenta and other nourishing membranes but which does not contribute to the ‘embryo proper’.