POU5F1 and teratoma: Similarly, Kim et al. [8] reported that transplanted porcine ntES cells did not form teratomas in immuno-compromised mice, indicating more studies are required to derive truly pluripotent ntES cells in pigs, most likely by the use of small molecule signaling inhibitors or forced expression of pluripotency-related genes, such as Oct4, Sox2, Klf4, and c-Myc [56–59].