Editing Embryonic stem cell (section)

===Adverse effects===
The major concern with the possible transplantation of ESCs into patients as therapies is their ability to form tumors including teratomas.<ref>{{cite journal|doi=10.1002/stem.37|pmid=19415771|title=Deconstructing Stem Cell Tumorigenicity: A Roadmap to Safe Regenerative Medicine|year=2009|last1=Knoepfler|first1=Paul S.|journal=Stem Cells|volume=27|issue=5|pages=1050–1056|pmc=2733374}}</ref>  Safety issues prompted the FDA to place a hold on the first ESC clinical trial, however no tumors were observed.

The main strategy to enhance the safety of ESCs for potential clinical use is to differentiate the ESCs into specific cell types (e.g. neurons, muscle, liver cells) that have reduced or eliminated ability to cause tumors. Following differentiation, the cells are subjected to sorting by [[flow cytometry]] for further purification. ESCs are predicted to be inherently safer than [[iPS cells]] created with genetically integrating [[viral vectors]] because they are not genetically modified with genes such as c-Myc that are linked to cancer. Nonetheless, ESCs express very high levels of the iPS inducing genes and these genes including Myc are essential for ESC self-renewal and pluripotency,<ref>{{cite journal|doi=10.1016/j.diff.2010.05.001|title=Myc maintains embryonic stem cell pluripotency and self-renewal|year=2010|last1=Varlakhanova|first1=Natalia V.|last2=Cotterman|first2=Rebecca F.|last3=Devries|first3=Wilhelmine N.|last4=Morgan|first4=Judy|last5=Donahue|first5=Leah Rae|last6=Murray|first6=Stephen|last7=Knowles|first7=Barbara B.|last8=Knoepfler|first8=Paul S.|journal=Differentiation|volume=80|pages=9–19|pmid=20537458|issue=1|pmc=2916696}}</ref> and potential strategies to improve safety by eliminating c-Myc expression are unlikely to preserve the cells' "stemness". However, N-myc and L-myc have been identified to induce iPS cells instead of c-myc with similar efficiency.<ref>{{cite journal|doi= 10.1016/j.stem.2007.12.001|pmid= 18371415|title= C-Myc is Dispensable for Direct Reprogramming of Mouse Fibroblasts|journal= Cell Stem Cell|volume= 2|issue= 1|pages= 10–12|year= 2008|last1= Wernig|first1= Marius|last2= Meissner|first2= Alexander|last3= Cassady|first3= John P|last4= Jaenisch|first4= Rudolf|doi-access= free}}</ref> Later protocols to induce pluripotency bypass these problems completely by using non-integrating RNA viral vectors such as [[sendai virus]] or [[mRNA]] transfection.