*The major concern with the potential clinical application of<unk> Cs is their propensity to form tumors. Much the same as ESC,<unk> Cs readily form teratoma when injected into immunodeficient mice. Teratoma formation is considered a major obstacle to stem-cell based regenerative medicine by the FDA. *A more recent study on motor functional recovery after spinal cord injuries in mice showed that after human-induced<unk>  stem<unk> s were transplanted into the mice, the<unk> s differentiated into three neural lineages in the spinal cord. The<unk> s stimulated regrowth of the damaged spinal cord, maintained myelination, and formed synapses. These positive outcomes were observed for over 112 days after the spinal cord injury, without tumor formation. Nevertheless, a follow-up study by the same group showed distinct clones of human-induced<unk>  stem<unk> s eventually formed tumors. *Since<unk> Cs can only be produced with high efficiency at this time using modifications, they are generally predicted to be less safe and more tumorigenic than hESC. All the genes that have been shown to promote<unk> C formation have also been linked to cancer in one way or another. Some of the genes are known oncogenes, including the members of the Myc family. While omitting Myc still allows for<unk> C formation, the efficiency is reduced up to 100 fold. *A non-genetic method of producing<unk> Cs has been demonstrated using recombinant proteins, but its efficiency was quite low. However, refinements to this methodology yielding higher efficiency may lead to production of safer<unk> Cs. Other approaches such as using adenovirus or plasmids are generally thought to be safer than retroviral methods. *An important area for future studies in the<unk> C field is directly testing<unk> C tumorigenicity using methods that mimic the approaches that would be used for regenerative medicine therapies. Such studies are crucial since<unk> Cs not only form teratoma, but also mice derived from<unk> Cs have a high incidence of death from malignant cancer. A 2010 paper was published in the journal Stem<unk> s indicating that<unk> s are far more tumorigenic than ESC, supporting the notion that<unk>  safety is a serious concern. *Concern regarding the immunogenicity of<unk> s arose in 2011 when Zhou et al. performed a study involving a teratoma formation assay and demonstrated that<unk> s produced an immune response strong enough to cause rejection of the<unk> s. When a similar procedure was performed on genetically equivalent ES<unk> s however, Zhou et al. found teratomas, which indicated that the<unk> s were tolerated by the immune system. In 2013, Araki et al. attempted to reproduce the conclusion obtained by Zhou et al. using a different procedure. They took<unk> s from a chimera that had been grown from<unk> C clones and a mouse embryo, this tissue was then transplanted into syngenic mice. They conducted a similar trial using ES<unk> s instead of<unk> C clone and compared the results. Findings indicate that there was no significant difference in the immunogenic response produced by the<unk> s and the ES<unk> s. Furthermore, Araki et al. reported little or no immunogenic response for both<unk>  lines. Thus, Araki et al. was unable to come to the same conclusion as Zhou et al. Recent achievements and future tasks for safe<unk> C-based<unk>  therapy are collected in the review of Okano et al.
Induced pluripotent stem cell
Fetal tissue implant
Neural stem cell
Neural tube
HybriCell
Embryoid body
Stem cell
Hematopoietic stem cell